Activins are thought to be involved in early differentiation processes during amphibian and avian development. Little is known, however, about the role of activins in early developmental stages of higher vertebrates. In order to study activin protein expression in early murine development we have prepared polyclonal antibodies against synthetic peptides corresponding to C-terminal sequences of the murine beta A and beta B activin subunits. Both antibodies recognized specifically activin A and activin B in different immunological assays including ELISA and immunoblotting. Using these antibodies in immunofluorescence experiments a good correlation between mRNA and protein expression of the beta subunits was observed in different in vitro model systems for early murine development. In addition, beta A and beta B subunit protein localization appeared to be different in 3.5- and 4.5-day mouse blastocysts. The results presented here are consistent with a role for different activin forms in the regulation of distinct processes in early murine development.