Cloning by limit dilution of an isolate of Leishmania tropica (LRC-L137) that is infective for mice resulted in 7 stable clones, only one of which was infective in BALB/c mice. Three of the non-infective clones that were examined for survival in BALB/c macrophages in vitro seemed to be killed more readily, suggesting failure to establish in macrophages as the basis for non-infectivity in vivo. Promastigotes from three non-infective clones and one infective clone were biosynthetically labelled or surface radioiodinated, and the detergent lysates were analyzed by 2-dimensional gel electrophoresis. The pattern of the radiolabelled cytoplasmic and membrane proteins of promastigotes from all L. tropica clones was similar, with minor differences. All clones as well as the uncloned population bound to the same extent to a series of lectins with galactose and N-acetylgalactosamine as specificities. They also bound in a solid-phase radioimmunoassay to 9 monoclonal antibodies raised against the uncloned L. tropica (LRC-L137). The genetic characterization of four L. tropica clones was attempted by analysis of their isolated kinetoplast DNA. The clones from two schizodemes since they possess kinetoplast DNAs which exhibit similar restriction endonuclease fingerprints and show extensive DNA sequence homology, suggesting that the four clones are closely related and that the non-infective variants may be derived from the infective presumptive parental clone L137-7-121. Further characterization of the clones of L. tropica should allow a better understanding of the genetic basis of parasite virulence in cutaneous leishmaniasis.