IgM-sensitized sheep E bearing guinea pig C4 and C3b (EAC43bgp) react as strongly with mouse spleen cell C3b receptors as do optimally prepared EACmo. Yet, when the C3b is converted to C3d by incubation with heated serum at low ionic strength, reactivity with mouse spleen cells is completely lost whereas binding to guinea pig spleen cells remains high. It is concluded on the basis of this and other evidence that EAC43bgp are a sensitive and specific indicator for mouse lymphocyte C3b receptors. Degradation of the C3b on these cells will not cause misleading binding to C3d receptors, as could occur with cells bearing mouse C3b.