Abstract: inhibition of immune precipitation by complament (IIPC) was measured by radioimmunoassay in 102 normal human sera and 93 SLE patients' sera. The low limit value of IIPC in the normal control was 84%. IIPC levels of sera in 63.4 percent of SLE patients were less than 84%. The mare serious the SLE patients' condition was, the lower IIPC value was. Sera with low concentrations of CH50, C3, and C4 and with high titer of anti-DNA antibody showed reduced IIPC levels.The IIPC level of serum can reflect SLE activity more sensitively.