Summary Mast cells can be separated from others in rat peritoneal washings by centrifugal elutriation with a purity comparable to that obtained by density gradient centrifugation. The principal of elutriation (countercurrent centrifugation) is to separate particles, having different sedimentation rates, suspended in a centripetal flow, balanced by the opposing centrifugal force. The yield of mast cells compared to the number in the original suspension was practically 100% under some conditions. An advantage of elutriation is that gradient material need not be removed from the fraction separated. The ability of the elutriation process to handle large volumes of cell suspensions in a continuous flow system would be advantageous for large-scale work.