The proliferative potential of human marrow cells stored at –170 C in 10% dimethyl-sulfoxide (DMSO) for periods of 3 days to over 9 months was examined in vitro using a suspension culture technique. The method employed tested the ability of the cells to synthesize hemoglobin in response to added erythropoietin (ESF). In 10 studies performed on marrow cells from 6 patients, hemoglobin synthesis in those cultures receiving exogenous ESF was increased 2–45 times that of control cultures. Sensitivity to radiation from an external source confirmed the proliferative nature of the response. The Do of the radiation effect averaged 68.25 rads, similar to other values determined for rapidly proliferating cell systems. These studies demonstrate that at least one population of precursor cells, the erythropoietin-responsive cells (ERC), is functionally preserved by the freezing technique employed.