Natural killer cell (NKC) plays an important role in anti-cancer, anti-virus and the regulation of immune reaction in human body. NK cell activities from the peripheral blood were determined with 51Cr release method from the human erythroleukemia cell line K562 in 61 patients, who were selected in the light of the standard of deficiency syndrome in TCM with insufficiency of the Kidney in which 12 kinds of diseases in western medicine were included, and 60 healthy subjects as control group. The patients with insufficiency of the Kidney showed significantly decrease in NK cell activity compared to the control group (25.29 +/- 10.20 vs 56.27 +/- 16.72, P less than 0.001). NK cell activities in 20 out of 60 patients, with deficiency of Kidney-Yang, were lower than 21 with deficiency or Kidney-Yin (19.38 +/- 7.44 vs 26.60 +/- 8.95, P less than 0.01) and 20 with deficiency of Kidney-Qi (19.38 +/- 7.44 vs 29.38 +/- 11.30, P less than 0.01), respectively, while there was no difference between the deficiency of Kidney-Qi (P greater than 0.05). The authors also found that NK cell activities of healthy subjects were strongly inhibited by plasma from 43 out of 61 patients. These results suggested that decreased NK cell activity is one of the common characters in patients with insufficiency of the Kidney in TCM and there are some inhibitory substances against NK cell activity in plasma of ones. The fact that NK cell activity of patients with deficiency of Kidney-Yang was the lowest among three kinds patients with insufficiency of the Kidney.(ABSTRACT TRUNCATED AT 250 WORDS)