Abstract: This article reports the NK cell activity and serum inhibition studies in SLE patients. When the NK activity of 25 SLE patients were compared with the NK activity in the age-and sex-matched controls, the results of NK activity were 10.79±7.34% and 32.69±9.46% respectively (P<0.001). Patients sera inhibition test was investigated. When the peripheral blood MNC of normal subjects were treated with 16 SLE sera for 4h, and were treated with 19 SLE sera for 20h before determining the NK activity, we found that the percentage of NK activity inhibited was 37.67±12.99% and 88.88±8.1% respectively. No correlations were observed between circulating immune complexes (IC) in SLE and sera inhibition. Other clinical parameters, such as PGE2 level of sera and anti-lymphocyte antibodies (ALA) also did not correlate with NK activity. We found that sera from SLE patients markedly inhibited the NK activity of normal MNC. Our data support the concept that serum factors in SLE patients reduce NK activity.