We previously described a simplified quantitative hemolytic assay for classical pathway (CP) hemolytic function in serum that has been shown to correlate with the 50% hemolytic complement (CH50) assay. In the present study, we used this assay to compare CP functions; plasma levels of C3, C4, and C3dg; and ratios of C3dg to C3 in healthy individuals and patients with systemic lupus erythematosus (SLE) or rheumatoid arthritis (RA) with different degrees of complement activation. A significant depression in CP function and levels of C4 and C3 and increased C3dg levels and C3dg/C3 ratios were observed in the SLE patients. In patients with RA, CP function was normal, whereas C3, C4, and C3dg levels and the C3dg/C3 ratio were elevated. The SLE results are compatible with systemic complement consumption, whereas the RA data suggest an acute-phase reaction with a normal C3 catabolic rate. To facilitate the handling of patient samples, we also developed a method to restore the hemolytic function of EDTA-plasma by transferring it to Veronal-buffered saline containing the thrombin inhibitor lepirudin. This process inhibits coagulation and enables complement activation, allowing a longer time lag between sample harvesting and testing. These results, combined with previous correlation studies, suggest that the CP hemolytic assay can effectively replace the CH50 assay for routine SLE differential diagnosis and monitoring of disease activity.