A temperature-sensitive strain of Sendai virus, HVJ-pi, showed little or no cytopathic effect and led to establishment of carrier cultures in several cell lines. By the use of this characteristic, L1210 leukemia cells persistently infected with HVJ-pi (L1210/c--HVJ-pi) was established, almost all of which were positively stained with fluorescent HVJ antibody. They are viable and grow almost equally as uninfected L1210 leukemia cells in vitro. Athymic nude mice (BALB/c, nu/nu), deficient of T-cells, died from intraperitoneal inoculation of L1210/c--HVJ-pi cells as well as by uninfected L1210 leukemia cells. However, viable L1210/c--HVJ-pi cells showed lower transplantability in normal syngeneic mice. This immunological mechanism of rejection was explained by the modification of cell surface membrane due to HVJ-pi infection. The mice which survived the inoculation of 10(5) L1210/c--HVJ-pi cells were able to reject 10(5) uninfected L1210 leukemia cells challenged subsequently. The induction of immune resistance was more prominent in (C57BL/6 x DBA/2)F1 mice or (BALB/c x DBA/2)F1 mice than in DBA/2 mice.