Susceptibility to Leishmania tropica was studied in Biozzi 'high' (Ab/H) and 'low' (Ab/L) responder mice of Selections I and III which, although originally selected with heterologous erythrocytes and Salmonella flagellar antigens respectively, both show non-specific separation of antibody responses. Ab/H Sel I produce only small lesions, even with the largest infecting dose, which heal after 2 months with minimal antibody responses. Specific delayed-type hypersensitivity (DTH) levels found in both are normal and equivalent. (Ab/H x Ab/L)F1 mice behave intermediately. A smaller inter-line difference is only detectable in Sel III with lower infecting doses. Both Ab/L and Ab/H mice given high doses develop chronic non-healing disease. These results imply that spontaneous resolution of cutaneous L. tropica lesions during the immune phase is independent of humoral antibody. While the great resistance and slower development of lesions found in Ab/L Sel I are entirely consistent with the macrophage 'hyperactivity' characteristic of the line, attempts to demonstrate this in vitro with comparisons of infected peritoneal exudate have been inconclusive so far.