In a continued effort to make interleukin-2/lymphocyte-activated killer (LAK) cell therapy safer and more efficacious for cancer patients, we examined methods of increasing the yields of cells obtained as a final product for reinfusion. Previously, the major cell loss occurred in the Ficoll-Hypaque gradient separation procedure used before cell culture. Therefore, we investigated the necessity of this step. Leukapheresis procedures (n = 105) from 40 different cancer patients showed that the resultant cell product is predominantly mononuclear (lymphocytes and monocytes; >97%) before the gradient purification step. The only cells observed to decrease in percentage as a result of the step were red blood cells (RBC:WBC ratio of 17:1 before purification to 1:3 after purification). Our study showed that the cytolytic potential of unpurified leukapheresis products against the LAK-sensitive line Daudi and the natural killer cell-sensitive line K562 was greater and that the patients received significantly more cells at times of reinfusion if the gradient separation step was eliminated. By additional experiments, we determined that autologous red blood cells enhance the generation of cytolytic LAK cells. This enhancement was greater if the red blood cells were in contact with the mononuclear cells during the time of cell culture. The elimination of the Ficoll-Hypaque purification step not only reduces the time and cost of the cell collection procedures, it also allows us to return to the patients greater numbers of cytolytic LAK cells following the activation period.