It has been postulated that platelet-derived growth factor and platelet factor 4 (PF4) are involved in the imbalance of the mechanism of medullar stroma maintenance which triggers off the bone marrow myelofibrotic process. In this work we compare the PF4 and the β-thromboglobulin (β-TG) and mitogenic activity in platelet-poor plasma (PPP) and platelet extracts (PE) from patients with myeloproliferative disorders (MPD) with those of secondary thrombocytosis (ST) and normal volunteers. Statistically significant differences were found between MPD and ST patients or controls, but none between ST and controls in all the parameters studied. Maximal differences in platelet-derived factors (PDFs) between MPD and control groups were found in polycythemia vera patients. However, the relationship between the presence of myelofibrosis and abnormal levels of β-TG, PF4 and mitogenic activity in PPP and PE was only observed in patients with agnogenic myeloid metaplasia (AMM). These results show that PDFs are specifically decreased in MPD platelets. Furthermore, no statistical correlation was found between PDFs and the number of platelets. However, other unknown factors or conditions would be necessary to develop myelofibrosis in MPD, which is present in A