Hemolytic activity of the total alternative complement pathway(ACH50), properdin activity(P activity) and Factor B activity(B activity) were measured in 64 systemic lupus erythematosus(SLE) patients using rabbit red blood cells. The following results were obtained. 1) ACH50, P activity and B activity were significantly lower in SLE patients than in normal subjects. 2) ACH50 correlated with CH50, C3, Factor B, B activity, C5 and P activity. P activity correlated only with ACH50. B activity correlated with ACH50, CH50, C3 and Factor B. 3) CH50, C4, C3, Factor B correlated with each other. 4) The degree of ACH50 reduction was less than that of CH50 reduction in the active stage of SLE. These findings suggest that the alternative complement pathway was activated mainly by the C3b positive feedback mechanism possibly due to triggering by immune complexes in vivo. The alternative complement pathway might play a part in defence mechanisms instead of decrease in the classical complement pathway in the active stage of SLE.