Serum factors which activated the alternative pathway of complement were detected in 10 of 26 patients with systemic lupus erythematosus (SLE), three of 18 patients with mixed connective tissue disease, one patient with scleroderma, and one with Sjögren's syndrome. This activation was detected by conversion of factor B and C3 into split products and by lysis of glutathione-sensitized human erythrocytes under conditions where classical pathway activation was blocked. The serum factors capable of activating the alternative pathway could be seperated into 7S and 19S-type molecules by sucrose density gradient ultracentrifugation. In two patients with SLE, serum factors were isolated by ion-exchange chromatography, preparative electrophoresis, and molecular sieve chromatography. The serum 7S factor was immunochemically identical to the C3 nephritic factor (C3NeF) of hypocomplementemic chronic glomerulonephritis. Antisera to other complement components failed to react with this purified material. The 7S factor was able to activate the alternative pathway in C2 deficient serum, and in normal human serum under conditions where classical pathway was blocked, but was unable to do so in sera depleted of factor B, factor D, and C3. The activity could be removed by antiserum to C3NeF. In contrast, the serum 19S activator of the alternative pathway was not reactive with antiserum to C3NeF. It had a fast gamma mobility on electrophoresis and the activity could be removed by absorption with anti-IgG and anti-IgM. It was suggested that the 19S factors could consist, in part, of immune complexes.