Summary. Traditional diagnostic criteria for primary thrombocythaemia (PT) remain essentially negative, aiming to exclude other myeloproliferative disorders and causes of reactive thrombocytosis (RT). It would be useful to have positive markers. We have examined several parameters to see how well they discriminate between PT and RT. Three groups of patients were studied: new, untreated PT (17), treated PT (12) and RT (17). Data consisted of: ESR, plasma fibrinogen, factor VIIIC, von Willebrand factor antigen (vWF:Ag), PDW, platelet nucleotide ratio (ATP:ADP) serum erythropoietin (Epo), ristocetin cofactor (vWF:RiCoF), multimeric structure of vWF, interleukin-6, evidence of clinical ischaemia and erythroid colony formation. Erythroid colonies were assayed in a serum-free system with the addition of Epo, IL3 or α-IFN to produce a discriminant function (DF) successfully used in the diagnosis of primary polycythaemia in an earlier study. Acute phase reactants (ESR, fibrinogen, VIIIC, vWF:Ag) and IL6 were the best discriminants, while PDW and serum Epo were less so. ATP:ADP and clinical ischaemia were nondiscriminatory in this study. Reduction in vWF:RiCof and in high molecular weight multimers were clearly associated with PT. Endogenous erythroid colonies were nondiscriminatory, but half the PT group and only one patient in the RT group obtained a DF suggestive of myeloproliferative disorder. Judicious use of a battery of tests may provide support for diagnosis of PT in difficult cases.