SummaryIn vitro lymphocyte transformation was studied in 24 subjects 12 months after vaccination with live Leishmania tropica major vaccine, in 10 normal control subjects and in three controls residing in an endemic area. The vaccines had lesions in various stages of clinical development. Lymphocytes from all the subjects were studied for their response to stimulation with L. tropica major promastigotes. Lymphocytes of vaccinated subjects responded to low concentrations of antigen whereas the lymphocyte response of the controls tended to be depressed by the same concentration of the antigen. Serum from each subject was subjected to a study of the humoral antibody titre against L. tropica major and L. donovani using indirect immunofluorescence. A humoral response to L. donovani was present in a majority of vaccinees who had developed a positive lesion whereas no such response was present in any of the controls. The data suggest that high humoral responses were accompanied by relatively low cell‐mediated responses and vice versa. No significant humoral response to L. tropica major could be demonstrated in any of the subjects. A combination of both the cell‐mediated and humoral mechanisms may participate in the immune response although their usefulness in the assessment of the protectivity of leishmania vaccines has not been establis