A method is described for isolation of antibody forming cells with the help of cluster formation. This was achieved by incubating spleen cells from immunized mice with sheep erythrocytes at 37°C. During incubation rabbit anti mouse immunoglobulin serum was present in order to facilitate the formation of clusters. Thereafter the clusters were isolated by velocity sedimentation. In this way suspensions were obtained containing more than 80% clusters. Some of the cells in the clusters still produced antibodies against sheep erythrocytes cultured for two days in vitro.