Previous reports have indicated that by mutagen treatment of mouse tumour cells in vitro it is possible to obtain at high frequency stable tumour cell variants that fail to form tumours in syngeneic mice because of increased immunogenicity. By analogy with these tumour cell variants, we examined whether variants with reduced virulence could be obtained by mutagen treatment of trypomastigotes derived from a Trypanosoma cruzi strain that was adapted to culture and produced lethal infections in DBA/2 mice at a dose of 5 X 10(4) parasites. A very large frequency of T. cruzi clones were obtained that failed to provoke an acute lethal infection after injection of 5 X 10(5) parasites. Most of these variants with reduced virulence (vir-) multiplied actively in normal mice until day 8 after injection. After that time the parasitaemia decreased gradually. For most variants a low level of residual parasitaemia persisted for more than 100 days. Unlike the situation encountered with mouse tumour cell variants it was not possible to demonstrate the presence of new antigens on the T. cruzi vir- variants. However, these variants seemed to have acquired an increased immunogenicity since they provoked the rejection of virulent parasites injected concomitantly. Mice that had been immunized with living vir- clones were protected against a challenge with a virulent clone derived from the original parasite population.