To evaluate the influence of sex steroids on immunity in 87 women on hormonal contraceptives, sensitization tests were performed with the contact allergen, 2,4-dinitrochlorobenzene (DNCB). Forty-five women were taking oral contraceptives of combined oestrogen/progestogen of the same brand (low oestrogenic and middle range progestogenic activity); 27 had received intramuscular injections of medroxyprogesterone acetate and 15 women were using a sequential pill. The results were compared with those of 44 women not taking contraceptive steroids and matched for age. In women on the combined pill and those who had received intramuscular progesterone, the mean DNCB reactivity was significantly increased (0.02 less than p less than 0.05 and 0.001 less than p less than 0.01, respectively), whereas women using a sequential pill proved to show a decreased skin reactivity in the oestrogenic phase of this pill. (p = 0.05). The changes observed proved to be specific, since no statistically significant differences in overall skin reactivity to irritants between the four groups of women could be observed. The clinical implications of these results have been considered.