The use of sterile connecting devices will permit up to 5-day storage of pooled platelet concentrates (PCs). However, there are no data evaluating long-term storage of PCs pooled from multiple donors. Four units of ABO-compatible or -incompatible PCs were pooled and stored in single 300-ml PL-732 storage bags for up to 5 days. Results of in vitro assays showed acceptable storage values regardless of the ABO types in the pool. Pool pH on Day 5 was 6.83 +/- 0.3 (mean +/- 1 SD). The in vitro storage characteristics were comparable to those of unpooled age-matched platelets reported previously from our laboratory. For in vivo studies, 4-unit pools of ABO-compatible random-donor PCs stored for up to 96 hours in 1000-ml PL-732 bags were transfused into patients who were thrombocytopenic due to bone marrow failure, and the correct count increments (CCI) were determined. In vivo results showed a mean 1-hour CCI of 11,368 +/- 5824 for the pooled stored platelets and 7819 +/- 5189 for unpooled controls (p greater than 0.05). To evaluate the possibility that passenger lymphocytes in the concentrates would generate mixed lymphocyte reactions (MLR) in the pooling bag during storage, lymphocytes were studied over 5 days of storage by the use of monoclonal antibodies against activated T-cell markers and by 3H thymidine uptake. Results failed to show evidence of either the generation of activated T-cell markers or the uptake of 3H thymidine.(ABSTRACT TRUNCATED AT 250 WORDS)