The effect of lymphocytosis-promoting factor (LPF) on antibody response in mice was estimated under different sets of experimental conditions. Four- and 6-week-old mice were intravenously inoculated with LPF. Three days later these mice were inoculated either intraperitoneally or intravenously with sheep red blood cell (SRBC) or human serum albumin (HSA) as an antigen. The adjuvant effect of LPF was demonstrated on antibody response in 6-week old mice to intraperitoneally inoculated SRBC but not to intravenously-inoculated one. When 4-week-old mice were immunized, hemagglutinin production in response to intraperitoneally inoculated SRBC was not enhanced by LPF. In addition, a rather suppressive effect of LPF at a comparatively high dose was demonstrated on hemagglutinin production in response to intravenously inoculated SRBC. Anti-HSA production was enhanced by inoculation of LPF in any combination of the mouse age and the route of antigen administration. These findings indicate that the adjuvant effect of LPF on antibody response in mice depends upon experimental conditions: the age of mice, the quality of antigen and the route of antigen administration used for immunization.