Colony formation by megakaryocytic progenitors (CFU-M) from the blood or bone marrow of 12 patients with essential thrombocythaemia was studied in vitro with the methyl cellulose assay. When the cultures were stimulated with plasma from a patient with aplastic anaemia and with phytohaemagglutinin stimulated conditioned medium (PHA-LCM), the patients showed a significant trend towards higher circulating CFU-M numbers when compared with the controls; three of the patients exceeded our normal range. In the bone marrow cultures there were no differences in the number or morphology of megakaryocytic colonies between the patients and the controls. When normal human plasma was the only source of colony stimulating activity, 11 out of 12 patients, but none of the controls, showed megakaryocytic colony formation. The same patients also had 'spontaneous' erythroid colony growth without the addition of exogenous erythropoietin into the cultures. Only one of the patients with essential thrombocythaemia (ET) had normal megakaryocytic and erythroid colony formation. The present study shows that, in most patients with ET 'spontaneous' CFU-M colony formation occurs in suboptimal culture conditions, a phenomenon obviously analogous to the spontaneous erythroid colony formation seen in the myeloproliferative disorders.