In order to study the effect of the Fu-Fang Si-Shen Yin (FFSSY) on cellular immunity, the natural killer (NK) cell activity of 60 patients with viral myocarditis, while the lymphocyte subsets consisted of OKT3, OKT4, OKT8, OKT4/OKT8 of other 20 patients were determined. The pretreatmental results were 25.25 +/- 0.46%, 59.11 +/- 8.72%, 30.72 +/- 6.94%, 29.72 +/- 5.35% and 1.04 +/- 0.24 respectively, which were significantly lower than that of the normal value (P < 0.01) except OKT8 However, after the treatment with FFSSY, NK cell activity and OKT3, OKT4, OKT4/OKT8 were elevated to 30.96 +/- 10.13% (P < 0.05), 62.33 +/- 7.78% (P < 0.05), 33.67 +/- 5.50% (P < 0.05) and 1.14 +/- 0.20 (P < 0.01) respectively. The Syndrome Differentiation of TCM indicated that NK cell activity decreased obviously in the Qi-Yin-Deficiency group, while T lymphocyte subsets changed significantly in the Qi-Deficiency group. There existed negative correlation between NK cell activity and OKT4/OKT8 (r = -0.60, P < 0.05). The patients with high NK cell activity in the early stage could have a favourable prognosis, while unfavourable with low NK cell activity.