Mobilized peripheral blood stem cells (PBSC) were collected in autologous plasma and acid-citrate-dextrose formula A (ACD-A) by leukaphereses using the CS3000 cell separator (Baxter) and stored at 4 degrees C in a refrigerator for 8 days. We have looked at the viability of the nucleated cells with the trypan blue test and the proliferation and differentiation capacity using a standardized progenitor cell cloning assay. The changes in viability, granulocyte-macrophage colony-forming units (CFU-GM), erythroid burst-forming units (BFU-E), and mixed-lineage colony-forming units (CFU-GEMM) were determined daily during the storage period. Viability was 90.8% (SD 8%) at day 0 and declined to a mean of 69.5% (SD 15.5%) at day 8. CFU-GM decreased to 47% (SD 28.7%), CFU-GEMM to 48% (SD 42.2%), and BFU-E to 40.1% (SD 18.4%) after 6 days. After 5 days of storage the mean viability was 79.7% (SD 17.8%), whereas the mean CFU-GM were 65.3% (SD 28.4%) the mean CFU-GEMM were 61.8% (SD 30.4%) and the mean BFU-E were 55.1% (SD 18.2%). At day 4 viability was still 82.5% (SD 17.0%), recovery of CFU-GM was 78.5% (SD 28.8%), recovery of CFU-GEMM was 70.7% (SD 40.4%) and recovery of BFU-E was 65.0% (SD 17.5%). These data show, that PBSC can be stored safely over at least 5 days at 4 degrees C while the patient receives high-dose chemotherapy.