The total yield and the proportions of Rhesus monkey peripheral blood mononuclear cells (PBMC) with detectable T4, T8, T11 and Ia markers that were purified by density gradient centrifugation through solutions of polyvinyl pyrrolidone (Percoll) were significantly greater than those found in PBMC suspensions obtained by using Ficoll-Hypaque. The percentages of recovery of PBMC and of cells expressing the above markers were lowest among PBMC isolated with a Ficoll-Hypaque mixture of density 1.077 and highest among PBMC obtained by using a solution of Percoll of density 1.077. Percoll of density 1.077 was also better than Ficoll-Hypaque in that it yielded PBMC with the lowest levels of erythrocyte (less than 1%) and granulocyte (less than 1%) contamination. The reduced levels of PBMC with detectable markers appeared to result from an effect of either Ficoll or Hypaque on the lymphocytes rather than from loss of lymphocyte subpopulations since higher proportions of cells bearing T4, T8, T11 and Ia were found when PBMC were isolated with a solution of Percoll which yielded a percentage of PBMC as low as that recovered with Ficoll-Hypaque. Altered marker expression by cells obtained with Ficoll-Hypaque was not seen with similarly prepared human PBMC.