Plasma samples from male CBA, BALB/c and DBA/2 mice were subjected to polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Bands of C6 hemolytic activity in the washed resolving gel were identified by means of an erythrocyte/agarose overlay gel. CBA plasma was found to contain two forms of C6, one with a molecular weight (Mr) of approximately 90 000 (type A) and one with a Mr of 100 000 (type B); CBA mice were thus designated C6: A+B+. BALB/c and DBA/2 plasma on the other hand contained only the low molecular weight type A C6 and were designated C6: A+B-. Flat bed isoelectric focusing followed by functional overlays showed that CBA plasma produced two sets of hemolytic bands (pI 5.25-5.4 and 5.0-5.2) whereas BALB/c and DBA/2 plasma produced only the more cathodal band set. The additional band set in CBA plasma corresponded to the high-Mr type B C6. Results of breeding experiments demonstrated that the possession of the high Mr type B C6 was inherited in the manner of an autosomal phenotypically dominant characteristic.