OBJECTIVE The present study was designed to characterize the clinical findings of patients with macroprolactinaemia (sustained hyperprolactinaemia where the predominant form of prolactin is of large molecular size) and to further assess the bioactivity and structure of big, big prolactin (BB-PRL). METHODS The patients with macroprolactinaemia were identified by the domperidone test and by gel filtration of their serum samples. The bioactivity of sera and fractions containing BB-PRL was evaluated and the fractions analysed by affinity chromatography and electrophoresis. METHODS Eleven patients with hyperprolactinaemia in whom BB-PRL was the predominant form (group 1) were studied. Sera were also examined from eight patients with prolactinomas (group 2). Thirty-one first-degree relatives from patients in group 1 were screened for macroprolactinaemia. METHODS Prolactin in the sera and fractions was measured by radioimmunoassay (RIA), immunoradiometric assay (IRMA) and Nb2 cell bioassay (Nb2 BA). The distribution of BB-PRL was also investigated after protein A Sepharose chromatography, immunoprecipitation, SDS-PAGE and Western blotting. RESULTS (1) Seven out of 11 patients in group 1 had galactorrhoea, menstrual disturbances or both; (2) the ratio Nb2 BA/RIA in whole serum was similar to the ratio found in group 2 patients; (3) the ratios Nb2 BA/RIA and Nb2 BA/IRMA in the BB-PRL fractions obtained after gel filtration were significantly lower than in whole serum (P < 0.003); (4) in group 1 patients, RIA estimates of PRL did not correlate either with Nb2 BA or with IRMA, while Nb2 BA and IRMA were correlated. In group 2 patients, RIA and Nb2 BA were correlated (r = 0.881; P = 0.02); (5) 24-86% of BB-PRL reacted as immunoglobulin-bound PRL as demonstrated by protein A Sepharose and by SDS-PAGE; (6) a high percentage of BB-PRL (54, 49 and 43%), was found in three of the serum samples obtained from 31 first-degree relatives of patients in group 1. CONCLUSIONS Macroprolactinaemia must be suspected not only in patients with asymptomatic hyperprolactinaemia but also in women with galactorrhoea and/or menstrual disturbances who have normal responses to PRL stimulating tests. Our results also suggest that the absence of symptoms in these women is not explained by a lower bioactivity of BB-PRL. Instead, we postulate that due to the high molecular weight, BB-PRL does not easily cross the capillary walls. During gel chromatography either a change in the structure of BB-PRL and/or a removal of substances which potentiate the bioactivity of PRL occurs, explaining the lower bioactivity of fractions containing BB-PRL in comparison with the serum. In this study we demonstrated that at least half (range 24-86%) of BB-PRL behaves as an immunoglobulin-bound PRL. Finally, we found that macroprolactinaemia was not genetically transmitted to first-degree relatives in the majority of the cases studied.