Fluorescent antibody technics were used to determine the localization and distribution of schistosomal antigen in tissue cells, using surgical samples of liver, spleen and lymph nodes. Samples obtained from 45 patients during spienectomy were Jrozen in dry-ice-acetone bath and sections 4 µ thick were cut on a cryostat microtome. Similar sections were fixed for routine histological and histochemical study. Serum samples were obtained from selected patients by skin test and conjugated with fuorescein isothiocyanate according to the modified method of Coons et al. For localizing the sites of antigen devosztion in tissue cells, direct and indirect tests were performed using fluorescein attached to the antibody (from patients serum). Thus the sites of antigen-antibody combination could be identified by their apple-green bright fluorescence. For confirmation of specifity inhibition, tests with non-conjugated serum from the same patients loere performed. The presence of antigen on the spleen and their relative amount were arbitrarily graded according to the frequence of cells involved, so that an attempt was made to correlate the immunological finding with the clinicai aspects of the patients. The patients were divided in 3 groups (0 to 19, 20 to 29 and 30 to 39 years of age) and were graded according to the severity of symptoms. A positive relaticmship was find in the first group (0-19 years) with a suggestive parallelism between the immunological and clinicai aspects. Pathological finds: - The spleen from patients of ali three groups showed hyperplasia of lymphatic follicles with a large germinal center and hyperplasia of the red pulp; the sites of antigen deposition were found by the fluorescent antibody technic in cells of the red pulp recognized as neutrophils and histiocytes, they were located in the sinusoids and Billroth cords and some in the trabeculae. The cells of the lymphatic centers did not show fluorescence. The pathological changes observed in the liver were characterized by inflamatory cells infiltrating the portal spaces and portal fibrosis characteristic as that described in Symmers "cirrhosis". They were located, immediately around the eggs, in several layers of histiocytes and neutrophils (some eosinophils), or a difuse infiltration of those cells were present in portal spaces with or without fibrosis suggesting no relationship to the eggs. The central area of the granuloma corresponding to the egg substance showed bright fluorescence, with the surrounding cells. When the egg was empty and the shell shrunked, the fluorescent material was found around the shell, and the number of fluorescent cells were more proeminent in the granuloma. The differents aspects related to the antigenic material corresponding to the egg substance, which showed positive to fluorescente antibody and the presence of fluorescent cells around, permit to describe a pattern of the granuloma mechanism: 1 st) The egg is full of fluorescent material without fluorescent cells around; 2 nd) the egg shell shows disrupture and fluorescent material starts to come out; some fluorescent cells are found just about the shell; 3 rd) the egg shell is contracted and empty, fluorescent material can be found around 4th) the eggs shell is reduced to a shrunked line (non specific fluorescent) with and a large number of fluorescent cells surrond the shell (full size granuloma); an area of non fluorescent cells, surronded by a zone of fluorescent cells; 5th). Finally, the granuloma is reduced to a shrunked shell surronded by a few fluorescent cells far from the center. Lynph nodes, as well as the spleen, did not show fluorescent cells in the limphatic follicles but they were very large and quite a number of eggs were located in the reticular cappilaries. The granuloimatous reactions did not show differences from those described in the liver. The authors suggest that the splenic changes, visualized by immunofluorescent technic were more uniform and can be representative to demonstrate the immunological stage of the disease. The liver, because of the location of eggs and worms, showed more reactive and variations could be found from one section to another.