Immunization of mice with two non‐cross‐reacting red cells, sheep and pigeon, gave rise to rosette‐forming spleen cells (RFC) capable of binding both types of erythrocytes. These double rosettes occurred in random bred mice and ininbred strains. The double RFC response vaned from strain to strain, from 6‐32% of the total RFC response. Serum taken from mice during the peak response was able to passively sensitize normal mice, resulting in a considerable number of RFC and double RFC recovered in their spleens. Strain differences in the number of double KFC determined following immunization were found to be related to the ability of individual strains to be passively sensitized with cytophilic antibody. The transfer could be performed in vivo or in vitro. It has also been shown that this cytophilic antibody can belong to the IgM or IgC class. This data indicates that cytophilic antibodies are responsible for all double RFC and for a significant varying number of the RFC in actively immunized mice depending upon the strain. Copyright © 1972 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim
