In an endemic area of Somalia both humoral and cell mediated immunity against Leishmania donovani was demonstrated in 246 inhabitants. In a study of 14 patients with active visceral leishmaniasis, we found that antibodies appear early in infection and that they are then demonstrable for a limited period only. Leishmanin positivity develops later and persists longer, but does not seem to be lifelong. The majority of the immunoreactive individuals were either sero- or leishmanin positive. This finding is in accord with the result obtained in recent experimental studies indicating a regulatory effect exerted on humoral and cell mediated immunity by different T lymphocyte subsets.