Spleen cell suspensions from mice undergoing a secondary response to sheep erythrocytes (SRBC) contained about one tenth as many specific antigen-binding, rosette-forming cells (RFC) when they had been washed at 37 degrees C instead of 4 degrees C before rosetting. This difference was correlated with the presence of IgG anti-SRBC antibody in the serum, and the 37 degrees C washings of immunised spleen cells could passively allergise non-immune spleen cells at 4 degrees C for specific rosette formation which was inhibitable by anti-mouse F(ab)2 serum. The RFC from actively immunised mice were lymphocytes and not macrophages by morphological and cytochemical criteria. It is suggested that the 37 degrees C-labile RFC are lymphocytes to which IgG antibodies bind in the cold. These data indicate that in the use of antigen-binding cell assays to monitor immunological responses, it is necessary to wash lymphocytes at 37 degrees C before testing.