Factors with growth hormone and lactogenic activities have been identified in serum samples and placental extracts of 9 species using a radioreceptor assay for prolactin or lactogens (RRA-PRL) and one for growth hormone-like activity (RRA-GH). The concentrations of placental lactogen (PL) and growth hormone-like activity (GHLA) in pregnancy were different in each species. The levels of PL began to rise at or before mid-pregnancy and either remained elevated until term (hamster, goat, sheep, monkey, and human), declined gradually after reaching peak concentrations just beyond mid-pregnancy (guinea pig), or had 2 peaks of activity (mouse, rat). The peak concentrations in different species ranged from 350 ng/ml in the cow to 45,000 ng/ml in monkey serum samples. The concentrations of GHLA were similar to PL levels in the guinea pig, goat, cow, and monkey but were lower than PL levels in the sheep and human. The maximal concentrations of PL and GHLA in placental extracts varied from 1 mug/g in guinea pigs to 200 mug/g in sheep. The ratio of lactogenic/growth hormone-like activity of both serum and placental extracts varied greatly among the various species. Gel filtration studies of serum samples revealed that PL and GHLA in the hamster, mouse and rat had different elution volumes, whereas, in the remaining species, the two activities co-eluted.