The proteinases of three species of Leishmania have been analysed by electrophoresis. Amastigotes of L. mexicana mexicana have several high-activity, low-Mr cysteine proteinases which are absent from log-phase promastigotes of L. m. mexicana and from all developmental stages of the other species analysed (L. donovani and L. major). Low-activity, low-Mr proteinases were present in populations of stationary-phase promastigotes of L. m. mexicana. All three species of Leishmania had higher Mr proteinases, a number of which showed developmental regulation, some of them being stage-specific. Significantly, at all stages of the life cycle in all three species a 68-kDa proteinase was apparent. In its size, sensitivity to inhibitors and ability to bind concanavalin A-agarose, this resembles the major surface protein thought to be present in all Leishmania species and which has recently been reported to possess proteinase activity in L. major promastigotes.