Passive transfer of a monoclonal antibody against progesterone produces a high incidence of maternal rejection in mice after recovery from antibody-induced infertility. To investigate the mechanisms involved in this reduction of maternal care, we have examined whether the effect is due to long-term exposure to antibody. Antibody was administered i.p. either on day 2 or day 17 of pregnancy. When a low dose (1.0 nmol) was given on day 2, pregnancy proceeded normally but 44.8% pups delivered at term were rejected compared with 12.7% in the control group. When a higher dose (4.5 nmol) of antibody was given on day 17, pregnancy continued normally to term and the rejection rate was 48.8% (control: 11.1%). When the same amount of antibody was injected after delivery (day 1 of lactation), no detrimental effect was found on subsequent maternal care to the young, the rejection rate being comparable between antibody-treated and control groups (5.3% vs 4.6%). To determine if the presence of antibody interfered with lactation or suckling, a bolus injection of 10 microCi [3H]H2O was given to mice treated at day 17 with antibody or saline. The levels of radioactivity present in both mothers and pups and the first 5-day pup growth curves showed identical patterns, indicating that milk availability and the suckling process were not affected. Crossfostering studies revealed that antibody-treated mothers rejected 25.5% of fostered pups compared with 8.5% found in the control females when antibody was administered on day 17 of pregnancy and the entire litters were crossfostered between the two groups immediately after delivery.(ABSTRACT TRUNCATED AT 250 WORDS)