SummaryIn the present study sera from twelve species have been compared to rat serum and ascitic fluid for their ability to support the growth of post‐implantation rat embryos. No alternative to rat serum or rat ascitic fluid was found. The basis for this apparent species specificity was further analysed. A marked improvement in embryonic growth was seen when rat serum was added to the heterologous serum prior to culture, whereas replenishment of a heterologous serum several times during culture had no effect. The experiments suggested that heterologous sera were nutritionally inadequate but not simply in energy source. Possible mechanisms for the specificity arc discussed.