Bone marrow collected from patients with hematologic malignancies were cryopreserved using DMSO as a cryoprotective agent. The growth kinetics of hemopoietic stem cells frozen to -196 degrees C were monitored immediately after thawing by the semisolid agar CFU-C assay and 2 different methods of cell reconstitution were compared. In the first way, thawed cells were plated after the removal of DMSO by washing the cell suspension, in the second, cell suspensions were cultured after a simple 1:1 dilution of DMSO with medium. The number of CFU-C per 2 X 10(5) cells plated was higher washing out the DMSO in all the groups studied. However, the absolute numbers of CFU-C contained in the whole ampoules after the freezing procedures was approximately the same with both methods. It is concluded that washing the cells only apparently yielded a better cloning efficiency, suggesting that such a procedure led to a higher mature nucleated cell loss with the consequence of a CFU-C concentration. This trend seems particularly evident in the AML and CML patients.