Seventeen patients with primary thrombocythemia (PT) were evaluated for in vitro bone marrow megakaryocyte progenitors (CFU-MK) using a plasma clot system. The aim of this study was to find out whether spontaneous growth of CFU-MK could be used in the diagnosis of PT. The number of CFU-MK was normal in 7 patients and reduced in 10 patients. In the absence of stimulating factor, CFU-MK grew spontaneously in 12 patients, while in 5 patients no spontaneous CFU-MK were observed. The mean plasma level of platelet factor 4 (PF4) was significantly higher (p < 0.05) in patients without spontaneous CFU-MK (59.8 ± 59.6 IU/ml; mean ± SD) compared to patients with (18.1 ± 20.7 IU/ml). The mean plasma level of β-thromboglobulin did not differ between patients with or without spontaneous CFU-MK. The β-thromboglobu-lin/PF4 ratio was significantly higher (p < 0.01) in patients with spontaneous CFU-MK (9.9 ± 7.1) compared to patients without (3.1 ± 1.4). These results suggest that PF4 could inhibit in vitro spontaneous growth of CFU-MK.