The purpose of this study was to use an immunization protocol with Moloney sarcoma virus (MSV-M) as active immunogen against exogenous and endogenous leukaemia. The s.c. route was chosen since it offered advantages over the i.m. route: the primary sarcomas were smaller, the regression faster, there were fewer recurrences and there was good persistent immunity. Strong protection was obtained against primary leukaemias induced by Friend leukaemia virus (FLV), Moloney leukaemia virus (MLV), Rauscher leukaemia virus (RLV), Precerutti-Law leukaemia virus (PLLV/T2), and H179A leukaemia virus. It was not possible to protect against leukaemia induced by Gross leukaemia virus (GLV). With transplantable leukaemias the results varied: partial protection was observed against H110 leukaemia (induced with human material) and R14 leukaemia (induced by X-irradiation) whilst no protection was obtained against P277 leukaemia (induced by Moloney leukaemia virus). As for spontaneous leukaemias, immunized BALB/c mice showed an increased incidence over the controls, while in F1 (Swiss x AKR) mice the incidence was similar but the latent period was shorter. Furthermore, in long-term observations the MSV-M-immunized mice showed an increased mortaltiy, which could be related to (1) new phenotypic mixtures between MSV-M and leukaemia viruses; (2) reactivation of MSV-M sarcoma-genesis with age, and (3) genotype susceptibility to MSV-M.