Despite recent advances in DNA-based genotyping, the microcytotoxicity test is still broadly used for the determination of human leukocyte class I antigens in patients as well as organ donors and also for the detection of HLA antibodies. Excellent purity and viability of peripheral blood mononuclear cells (PBMC) are essential for reliable HLA typing results. Background staining and cell loss can contribute to impaired typing results or even cause misinterpretations. A novel isolation procedure using cell preparation tubes (CPT) with prefilled Ficoll was compared with the standard Ficoll gradient. We determined the recovery, purity, and viability of the PBMC after several periods of storage. Finally, the isolated cells were used for HLA class I typing, and background reactivities were scored. By using the CPT method, the recovery of PBMC was significantly higher than recovery with the standard technique (P  0.001). Contamination by granulocytes increased considerably during the storage time for the standard protocol, whereas purity remained stable when CPT were used (P  0.001). With both methods, lymphocyte viability declined markedly over time. We found significantly more dead cells by using the CPT methods. Due to high background scores, HLA typing was impossible after 48 h. The isolation of PBMC by the CPT method resulted in a higher yield and improved purity compared to those obtained with the standard gradient technique. The decreasing viability after 48 h limits the use of both methods for HLA typing and HLA antibody screening.