A medium conditioned by leukocytes in the presence of phytohemagglutinin (PHA-LCM) promotes the growth of human multilineage hemopoietic progenitors (CFU-GEMMT) which form mixed hemopoietic colonies in culture containing granulocytes, erythroblasts, megakaryocytes, macrophages, and T-lymphocytes. PHA-LCM derived from six HLA-typed patients with idiopathic hemochromatosis and from six normal individuals were tested for growth-promoting activities for multilineage hemopoietic colony formation. Four out of six conditioned media obtained from patients with hemochromatosis supported mixed hemopoietic colony formation, as did four of six conditioned media from normal HLA-typed volunteers. Active PHA-LCM preparations from patients with hemochromatosis were similar with respect to the number and size of mixed colonies and their cellular composition when compared with conditioned media obtained from volunteers. The study indicates that no link exists between the HLA phenotype of a particular donor and the predictability of obtaining an active PHA-LCM promoting multilineage hemopoietic colony formation. PHA-LCM derived from patients with hemochromatosis had no advantage with respect to stimulatory activity for mixed colony formation when compared with conditioned media obtained from healthy volunteers.