We have established single-cell culture for human mixed hemopoietic colonies using a micromanipulator. Mononuclear cells from human umbilical cord blood were cultured at a concentration of 1 X 10(4) cells per milliliter in methylcellulose medium containing medium conditioned by phytohemagglutinin-stimulated leukocytes and erythropoietin. It was possible to identify the single hemopoietic progenitors in situ in methylcellulose culture on the basis of unique morphology and migratory ability after 36 to 60 hours of incubation. Candidate single hemopoietic progenitors from methylcellulose medium were individually micromanipulated to secondary culture dishes and cultured for an additional ten to 14 days. The colonies derived from the single progenitors were individually picked and stained with May- Grunwald-Giemsa for analyses of the cellular composition. A total of 288 single cells were individually transferred to second dishes. Then 186 single cells produced secondary colonies consisting of cells in one to five different lineages. A total of 39 single cells produced mixed hemopoietic colonies consisting of cells in two, three, four, and five different lineages. There were eight types of colonies revealing two different lineages, ie, neutrophil (n)-erythrocyte (E), macrophage (m)- E, m-megakaryocyte (M), eosinophil (e)-basophil (b), eE, bE, bM, and EM lineages. Three types of colonies consisting of cells in three lineages were also seen, ie, nmM, nbE, and ebE. There were six types of colonies consisting of cells in four lineages, ie, nmbM, nmEM, nebE, mebM, and meEM. One type of colony consisted of cells in five different lineages (nmbEM). These results indicate the single-cell origin of human mixed hemopoietic colonies expressing various combinations of cell lineages. It also provides experimental data in support of stochastic mechanisms of stem cell differentiation.