Pregnancy was blocked by anti-progesterone monoclonal antibody in two inbred (BALB/cJ, CBA/Ca) but to a lesser degree in an F1 hybrid (CBA/Ca male X BALB/cJ female) or an outbred (Tuck's no. 1) stock of mice when antibody was injected intraperitoneally (i.p.) at 32 h post coitum (p.c.) using a dosage of 9.5-10.9 nmol. This different antifertility effect could not be explained solely by altered tubal transport in inbred mice since the rate of transport was slightly accelerated in one stock (BALB/c) but not in another (CBA). In crossbred mice tubal transport was not significantly altered by antibody treatment. At Day 3 (54-58 h p.c.), the majority of embryos in control mice were at the 4-cell and 8-cell to morula stages in inbred and crossbred stock, respectively, but after antibody treatment they were mainly at the 4-cell stage in all 4 stocks. At Day 4 (78-82 h p.c.) the majority of embryos in control females had reached the blastocyst stage in all stocks, whereas after antibody treatment they had reached this stage in crossbred stock and relatively few had progressed so far in inbred stock. The results indicate that there are two events in early gestation which are susceptible to passive immunization with anti-progesterone monoclonal antibody. The first of these occurs during cleavage shortly after the 4-cell stage when embryo development was arrested in two inbred stocks of mice. Antibody effects on cleavage were not direct since embryos cultured in the presence of high concentrations of antibody, or antibody saturated with progesterone, continued to develop in the normal way and formed blastocysts. The second event is the onset of implantation, an effect also influenced by genotype. The decidual cell reaction induced by intraluminal oil injection was blocked by antibody injected at 8 or 32 h p.c. in BALB/c females, but only when injected at 8 h, and not at 32 h p.c., in F1 hybrid females. The results show that there is a greater resistance in two crossbred stocks compared with two inbred stocks to the effects of passive immunization against progesterone in early pregnancy.