Abstract The EAC generating capacities of fresh and stored BALB/c, Swiss, C3HeB/FeJ, and C3H/HeJ mouse sera were investigated in a rosette test using spleen cells of each of the serum donors as indicator cells. With BALB/c spleen cells a decreased ability of both stored and fresh C3H/HeJ serum to generate functional EAC could be registered, whereas with C3HeB/FeJ spleen cells such was the case only with stored serum. Swiss spleen cells did not discriminate between the capacities of the four sera and with C3H spleen cells both C3H sera tended to be less effective than BALB/c and Swiss serum. This indicates that C3H/HeJ serum is able to generate EAC but with a somewhat different disposition of cell-bound C molecules. Our results suggest that the “defect” in C3H/HeJ serum is localized in C 4, C 3, or rather in some C-associated inhibiting system as C 4-binding protein or the C 3b inactivating system. A possible relationship of the “defective” serum factor and the defective Lps gene product is suggested.