Forty adult subjects were studied with the aim of establishing positive diagnostic criteria in primary proliferative polycythaemia (polycythaemia vera, PPP). These comprised 14 patients with PPP, eight secondary polycythaemia (SP), five idiopathic erythrocytosis, and 13 normal subjects, classified under standard criteria following comprehensive investigation for causes of SP. Erythroid colony formation from peripheral blood in a serum-free system was assayed with the addition of recombinant human erythropoietin (Epo), interleukin 3 (IL3), or α-interferon (α-IFN). The differential sensitivity of primitive and mature progenitors (BFU-E) was assessed by counting the number of clusters (‘sub-colonies’) comprising each erythroid burst. ‘Endogenous’erythroid colonies were found in both PPP (56%) and controls (17%). In Epo containing cultures, the mean number of clusters per burst was lower in PPP than controls, and the percentage of small (≦ 8 clusters) bursts was higher. In PPP primitive BFU-E demonstrated greater dependence on IL3 than controls, and mature BFU-E greater inhibition by α-IFN. These findings suggest an abnormal response to several growth factors, rather than dysfunction of a single growth factor receptor. Regression analysis of these data defined a discriminant of high diagnostic sensitivity and specificity. This discriminant accurately predicted diagnosis in a further nine polycythaemic patients.