SummaryAfter severalin vitrotreatments of a virulent population ofLeishmania majorwith the mutagen,N‐methyl‐N'‐nitro‐N‐nitrosoguanidine (MNNG), five clones (vir‐) were obtained that did not produce cutaneous lesions after subcutaneous injection of 106promastigotes. All the control clones (vir+) obtained from the non‐mutagenized parasite population produced progressive cutaneous lesions with as few as 103parasites. Late lesions were observed occasionally after injection of 107vir‐parasites. These late lesions appeared to result from the selection of virulent revertants, since parasites isolated from these lesions produced progressive lesions in BALB/c mice almost as readily as the control parasites.Two vir‐clones, one vir+clone and one revertant clone were examined for survival in BALB/c macrophagesin vitro.All clones were taken up by the macrophages and transformed into amastigotes. However, vir‐clones failed to multiply inside the macrophages.A vir‐clone was found to protect mice against a subsequent challen