One hundred and sixty seven cases of pituitary adenoma were analysed using the immunocytochemical method of the Avidin-Biotin Complex (ABC), described by Hsu et al. (1981). Six pituitary anti-hormones were utilized: anti-prolactin (aPRL») at a 1:1,500 dilution; anti-growth hormone (aHGH) at a 1:4,000 dilution; anti-adrenocorticotro-phic hormone (aACTH) at a 1:3,000 dilution; anti-thyrothrophic hormone (aTSH) at a 1:3,000 dilution; anti-luteinizing hormone (aLH) at a 1:1,000 dilution; and a anti_follicle-stimulating hormone (aFSH) at a 1:300 dilution. Incubation period was 14 to 16 hours at 4°C. The survey of clinical, laboratory and radiological data of cases of pituitary adenomas was performed (after reading- the stained slides using the immunocytochemical method. Of the 167 cases of pituitary adenomas, 136 (81,4%) disclosed a positive immuno-reaction to one or more anti-hormones, and the positivity index of neoplastic cells varied from 1 to 90%. The immunoreaction was positive exclusively to one anti-hormone in 80