Different clones and subpopulations of clones of Leishmania infantum were analyzed for their infective potential in vitro. Infectivity for macrophage-like cells (P388D1) and the response to a challenge with normal human serum enabled a clear differentiation between infective and non/low-infective populations. The results were confirmed by determination of parasite burdens in the spleens of infected golden hamsters. Long-term cultivation assays in vitro were used to test the influence of different cultivation conditions on the infectivity of promastigotes. An originally infective clone (LEM 768-A/ST) lost its infectivity during these assays but could regain it after the cultivation conditions had been changed. In addition, an originally non/low-infective clone (LEM 287-D/ST) could be forced to produce highly infective promastigotes. Infective cells were found only among stationary-phase promastigotes, i.e. after the cultures had reached a maximal number of cells per milliliter and the cell volume had clearly decreased.