The persistence of anti-HBs antibodies elicited in response to vaccination with a recombinant yeast-derived hepatitis B vaccine was examined. Three years after vaccination following two different schedules (0, 1, 6 months and 0, 1, 2, 12 months), anti-HBs antibody titres obtained after vaccination with the yeast-derived vaccine were similar to those observed using a plasma-derived vaccine. At this time, nearly 100% of all vaccinees had anti-HBs antibody titres over the protective limit of 10 mIU ml-1. These results suggest that the yeast-derived vaccine will confer protection against hepatitis B infection for about the same length of time as the plasma-derived vaccine.