Summary:Lymphocyte fractions obtained by Percoll density gradient centrifugation were assayed for their cytotoxic activities both in NK and ADCC cytotoxic systems. Furthermore, the release of a mediator, platelet slowing factor from the same subsets, was evaluated. Different mitogens (phytohemagglutinin, concanavalin A and lipid A) have been used for this purpose. The maximum of cytotoxic activities was recovered in E+ cell-enriched fractions. On the other hand, lipid A was able to stimulate the PSF production from enriched B cells (fraction I); phytohemagglutinin induced the release of PSF from E+ fractions (II and III) and concanavalin A from fraction II only. Our results suggest that Percoll density gradient is a very reproducible method in obtaining lymphocyte subsets enriched in cells mediating either cytotoxic activities or lymphokine release.