An in vitro bioassay for plasma PRL-releasing factor-like activity has been developed. The method is a three-phase methanol extraction of plasma with extracts of 1.0 ml plasma adjusted to a final volume of 50 microliter. Single 50-microliter aliquots of extract were incubated in 1.0 ml Krebs-Ringer phosphate (KRP) buffer with one rat hemipituitary after a 1-h preincubation. Samples were obtained basally and 30 min after addition of the extract. During each set of incubations, a parallel series of hemipituitaries was incubated in KRP alone. The total nanograms of rat PRL released per mg pituitary tissue during the initial 30 min after preincubation was calculated for all studies. The mean quantity released in KRP alone was considered basal and was subtracted from values obtained during incubation with plasma extracts. The quantity remaining was considered PRL-releasing activity (PRA) of plasma, expressed as nanograms of rat PRL released per mg pituitary. The PRA in plasma from 13 patients with the amenorrhea-galactorrhea syndrome was 132 +/- 17 ng/mg pituitary (X +/- SE), which was significantly greater (P less than 0.001) than the PRA in plasma from eight matched controls [31 +/- 10 ng/mg pituitary (X +/- SE)]. The patients' individual PRL levels were elevated (range, 48-248 ng/ml), and when compared to the PRA in the samples, a highly significant (P less than 0.001) positive correlation evolved. These results indicate that a circulating PRL-releasing factor-like material present in normal plasma is higher in plasma from hyperprolactinemic patients in direct relationship to the PRL concentration. It is possible that this material is related to the pathogenesis of PRL-secreting pituitary disorders.