Colonies of eosinophils, neutrophils, monocytes, and macrophages were grown in vitro from peripheral leukocytes of normal individuals. The concentration of circulating nucleated cells capable of giving rise to such colonies is considerably less than in marrow. In blood obtained from six healthy adults, between 4.5 and 21.6 colonies were observed per 106 nucleated cells plated. Growth characteristics of these colonies were similar to those arising from marrow cells. Colony formation was observed after 6-10 days of incubation and increased to a maximum size of 200-1000 cells after 18-20 days and then began to undergo degeneration. Colonies of eosinophils were the most frequent cell type observed. Neutrophil and monocyte colonies were the next most common and occurred with about equal frequency; colonies of mixed cell type were less frequent than the above three types. These colonies were mainly observed at about 3 weeks of incubation. Colonies examined after 4 weeks of incubation consisted primarily of large histiocytes and macrophages. Studies with tritiated thymidine (3H-TDR) revealed that all colonies examined contained labeled cells, indicating that these colonies arose from cell proliferation.