We have investigated the properties of mobilized, cryopreserved, peripheral blood stem cells (PBSC), collected by leukapheresis over a period of 5 days, from eight myeloma patients in clinical remission. Cells were mobilized by treatment with cyclophosphamide and granulocyte colony-stimulating factor (G-CSF), and each day's collection was evaluated for its content of CD34+ cells, colony-forming units granulocyte/macrophage (CFU-GM), and plastic-adherent pre-CFU-GM. Peak values for these three parameters were observed at different times in different patients. There was no correlation between CD34+ content and CFU-GM, but there was some (r = 0.65) between CD34 numbers and colonies generated from a delta assay initiated using plastic-adherent pre-CFU-GM. In suspension cultures, the cells grew exponentially for 50 days. Thereafter, they did not divide, although they remained viable in culture for up to 1 month longer. Suspension cultures of PBSC grown with interleukin-3 (IL-3) displayed a predominantly myelomonocytic phenotype, but some megakaryocytes and erythroid cells were observed consistently. These results indicate that pre-CFU-GM in PBSC collections are capable of generating large numbers of clonogenic progeny in liquid culture and are capable of producing multiple lineages of differentiation.