Murine L cell fibroblasts, and derivatives were found to rosette with sheep red blood cells (SRBC). Primary fibroblast explants from the parent murine strain, C3H, did not possess this potential. No rosettes were observed with primary fibroblast explants from C57BL and B10Br mice, with a human fetal lung fibroblast, with baby hamster fibroblasts or their polyoma transormed derivative, or with a cell line, 1T-22, derived from BALB/c mice. Hybridization of 1T-22 and L cells, by Sendai virus-mediated cell fusion, suppressed the rosette potential of the L cell parent. The receptor for SRBC on L cells appears to result from the expression of a recessive characteristic.