Abstract Rosette formation between human lymphocytes and mouse red blood cells (MRBC) was examined as a marker for B lymphocytes and as a method of B lymphocyte separation. A small proportion of lymphocytes formed spontaneous rosettes with MRBC (mean value 6%) and the number was considerably increased by pretreating the lymphocytes with neuraminidase (mean value 16%). Double marker tests demonstrated that lymphocytes forming MRBC rosettes were immunoglobulin (Ig) bearing cells, with a high proportion of IgM bearing cells, but not all Ig bearing cells formed MRBC rosettes. Lymphocyte populations enriched with T or B lymphocytes, by SRBC rosette sedimentation and nylon column filtration, gave values for MRBC rosettes consistent with a subpopulation of Ig bearing cells. Separation of MRBC rosette-forming cells gave a relatively poor degree of separation and rerosetting with MRBC produced variable results.