RIA and bioassay (BA) estimates of the concentration of PRL ([PRL]) in rat serum were compared using homologous assay systems. A rat mammary gland organ culture BA was developed which allowed objective and quantitative measurement of serum [PRL]. the serum samples were tested at two doses against three doses of the RP-1 rat PRL standard and the responses were quantified by a histometric procedure. The index of precision of the BA ranged from 0.1--0.4 and the secretory response was not affected by physiological concentrations of rat GH, T3, or by gonadal steroids. Serum samples taken from female rats during the estrous cycle, from suckled lactators, from rats bearing a PRL-secreting pituitary tumor, and from females that were treated with perphenazine were assayed. A highly significant correlation (r = 0.99) was obtained between the two assay methods with 21 samples. However, the regression coefficient was 4.20 +/- 0.55.Thus, although the two assays are highly correlated for measuring serum [PRL], the RIA measures only about 25% of the hormone that is detected by the mammary gland organ culture BA.