The in vitro cultures of haematopoietic progenitors have been reported to be useful in the diagnosis of myeloproliferative disorders since the so-called endogenous erythroid and megakaryocyte colony formation has, in most studies, been found in these diseases. In order to know their value as diagnostic criteria in essential thrombocythaemia (ET) we have studied megakaryocyte (with and without phytohaemagglutinin-stimulated leucocyte conditioned medium) and erythroid (with and without erythropoietin) colony formation in vitro by progenitors from blood in 60 patients with ET and in ten with reactive thrombocytosis (RT) using the methyl-cellulose assay. Out of 60 ET patients endogenous megakaryocyte colony growth was observed in 38 (63%) and endogenous erythroid growth in 42 (70%). None of the patients with RT or any of the controls showed either type of endogenous growth. Fifty-five (91%) of the patients with ET showed megakaryocyte and/or erythroid endogenous colony formation whereas five (9%) did not have any kind of endogenous colonies, although cultures were performed sequentially. In conclusion, a positive endogenous megakaryocyte and/or erythroid colony growth from blood is a frequent and characteristic finding in ET patients and should be used as a useful marker in this disease.