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.