This study based on LINEWEAVER & BURK's (1934) equation was made to verify both competitive or not effect of Mg2_f in absorption of Ca2+ ion. Solution of CaCl2 was traced with 45Ca. Calcium in different concentration was added to excised leaf tissue of Coffee-tree (Coffea arabica L.) in presence or not of magnesium, according to SMITH & EPSTEIN's (1964a.; 1964b.) technique. Special conduction in statistics was applied. Magnesium ion compete with calcium ion to active centers of metabolism transport.