Six patients with Philadelphia positive chronic myeloid leukaemia (CML) were treated with single high doses of busulphan. The action of busulphan on the in vivo kinetics of circulating progenitor cells (colony forming cells) was measured using an agar culture system which involved scoring of total colonies and clusters at 7 days and of granulocyte, monocyte and eosinophil colonies at 14 days. High dose busulphan was found to be effective in suppressing circulating granulocyte, monocyte and eosinophil progenitor cells. The effect of busulphan on progenitor cells was rapid and their levels fell by at least 85% within five days. By contrast, the white blood cells fell by only 9% and the platelets fell by 10% over this time. Subsequently, the white cell count and platelet count fell to near normal levels. The progenitor cell levels began to rise again at a mean of 35 days following busulphan treatment and the white blood cells at a mean of 39 days in four patients. One patient remained in haematological remission for six months following 100 mg of busulphan with less than 1 progenitor per 5 x 10(5) peripheral blood nucleated cells. One patient in myeloblastic transformation had a previously not described culture pattern consisting of a high cluster to colony ratio at 7 days and of an increased number of predominantly eosinophilic colonies at 14 days. There was no significant fall in progenitor cell levels following busulphan and this patient died.