Two sera from unrelated individuals, each lacking C8 activity, were examined by Western blot analysis. Using antisera raised against whole C8, the two sera are shown to lack the C8 beta chain, indicating a C8 beta deficiency, which is frequently observed in cases of dysfunctional C8. In contrast, by means of a specific anti-C8-beta antiserum, a C8 beta-like polypeptide chain of apparently identical molecular weight compared to normal C8 beta was detected. Digestion of normal and dysfunctional C8 beta with Staphylococcus aureus V8 protease revealed distinct differences in the enzymatic digestion pattern. We conclude that the dysfunction in the C8 protein in these two patients resides in the dysfunctional C8 beta chain, and that this form of C8 deficiency is distinct from C8 deficiencies previously reported, in which one or both C8 subunits are lacking.