SUMMARY An in-vitro bioassay for prolactin has been devised, based on the stimulation of casein synthesis in a mouse mammary gland preparation. Dispersed mammary gland cells were superior to intact explants for this purpose. Casein synthesis by dispersed cells was stimulated by added prolactin, and a linear log dose—response curve was established (for the range 5–20 μg prolactin/ml). The precision of the assay was high (λ = 0·10–0·15). Sensitivity was rather low, but could be improved by increasing the concentration of amino acids in the medium. The response to prolactin was not influenced by thyroxine, adrenocorticotrophin or oestradiol, but thyrotrophin appeared to inhibit it slightly. Both human placental lactogen and bovine growth hormone showed lactogenic activity in the assay.