Clonogenic cultures support the proliferation of haemopoietic cells into colonies of differentiated progeny. Using these techniques it has been established that normal haemopoietic progenitors are dependent upon growth factors for their survival, proliferation and differentiation in vitro. However, progenitors from patients with polycythaemia vera (PV) generate erythroid colonies in cultures deprived of exogenous erythropoietin. These have been termed endogenous erythroid colonies (EEC). Recently, endogenous megakaryocytic colonies (EMC), those arising in assays without megakaryocyte growth factors, have been reported, particularly in patients with primary thrombocythaemia (PT). Many investigators have found an EEC positive culture to have 100% diagnostic specificity and sensitivity for PV. Similarly, EMC have been shown by some to be an unequivocal marker of PT. Accordingly, clonogenic assays for EEC and EMC have been advocated as diagnostic markers of PV and PT. However, the specificity of these assays is not universally attested to as there are some reports of EEC in patients with secondary polycythaemia and of EEC and EMC in normal subjects. Thus, for diagnostic use, EEC and EMC assays must be exhaustively validated for specificity using clinically appropriate controls. Furthermore, clonogenic culture techniques are not amenable to external quality assurance, are technically demanding and are unlikely to be available in most haematology laboratories. These considerations must be taken into account when assigning weighting to EEC and EMC assays as diagnostic criteria in myeloproliferative disorders.