Human pluripotent hemopoietic progenitors (CFU-GEMM) from mixed colonies when cultured in methylcellulose or agar in the presence of erythropoietin and PHA-LCM. The observed frequency is low and varies for different individuals between 0 and 4 mixed colonies/O5 mononuclear cells. CFU-GEMM do not adhere to glass or plastic surfaces; their density is less than 1.077 g/ml, and their sedimentation velocity profile peaks at 4.5 mm/h. They do not form rosettes with sheep red blood cells. These physical parameters can be used preparatively to enrich for CFU-GEMM to facilitate assessment of their biological properties such as cycle state analysis and measurement of self-renewal capacity. Preliminary information suggests that some CFU-GEMM are capable of self-replication. Cycle state data are available for a larger number of patients with various clinical conditions. CFU-GEMM were found to be quiescent under steady-state conditions. They proliferate actively during bone marrow regeneration and in stem cell disorders like Polycythemia rubra vera or CML. These changes in cycle state activity were not reflected in numerical alterations of CFU-GEMM. It was thus concluded that the assay may be used more meaningfully to assess biological properties of human pluripotent progenitors rather than their frequency.