Intra-dermal skin tests (IST) were applied with candidin (1:100) in 37 subjects and repeated 10 days later. The induration diameter of the second injection (read at 48 h) was greater (p less than 10(-8]. A correlation (r = 0.76, p less than 0.001) appears between the first and second induration diameters. In 17 of these patients (randomly distributed) we measured the absolute number of mononuclear cells bearing receptors of Fc IgG (RFc) before the first and before the second injection. The number of RFc was greater (p less than 0.02) after the second injection. It may be possible that candidin testing increase T suppressor cells. In 20 other subjects, we observed an increase of induration diameter (p less than 0.05) after a second IST of PHA (2 micrograms) applied 15 days after the first injection. Repeated IST with candidin and PHA induce a very high level of booster phenomenon. This fact may be kept in mind when interpreting serial IST in immunodeficiencies.