The conditions for efficient introduction of the contents of liposomes into cells were examined using fragment A of diphtheria toxin (DA) as a marker; one molecule of DA can kill a cell when introduced into the cytoplasm. Liposomes containing DA (DA liposomes) were toxic to cells treated with HVJ (Sendai virus) at 4 degrees C just before exposure to DA liposomes at 37 degrees C, but were not toxic to untreated cells. This toxicity was temperature-dependent. DA outside of liposomes was not toxic to HVJ-treated cells. Results also showed that liposomes could fuse with HVJ at 37 degrees but not at 4 degrees C and that liposomes preincubated with HVJ at 37 degrees C could associate with cells. DA liposomes preincubated with HVJ at 37 degrees C were highly toxic to cells. This toxicity was dependent on the duration of preincubation with HVJ and the dose of HVJ. When plasmid DNA coded herpes simplex virus thymidine kinase was trapped in liposomes and fused with Ltk- cells with HVJ, the thymidine kinase activity was expressed in about 10% of the cells. These data show that naked liposomes fuse efficiently with cells with HVJ and that the contents of the liposomes can be introduced into the cytoplasm 100-10 000 times more efficiently by treatment of the cells or liposomes with HVJ.