Peripheral blood progenitor cells (PBPC) harvested for autologous transplantation are usually cryopreserved within 2-4 h of collection. However, there are conditions in which it would be useful to freeze PBPC after a liquid storage period. This study was performed to evaluate whether prior storage for just 24 h damages frozen PBPC. First, leukapheresis products were obtained from 9 patients and divided into three fractions. The first fraction was frozen within 2 h and used as a control. The second and the third fractions were stored either at 4 degrees C or at 22 degrees C for 24 h before freezing. Cell counts, CFU-GM values, and pH were studied after collection, after storage, and after cryopreservation. Similar results were obtained at 4 degrees C and 22 degrees C. However, pH decreased most markedly at 22 degrees C. Mean postcryopreservation CFU-GM recoveries were not significantly different and were, respectively, 74.03% (control), 96.39% (4 degrees C), and 80.33% (22 degrees C). These observations were confirmed in an additional study of frozen PBPC collected from 12 patients and previously stored for 24 h at 4 degrees C. These data indicate that blood progenitors may be stored for 24 h and subsequently frozen without quantitative and qualitative impairment.