OBJECTIVE It was reported that the symptoms of tumor patients may be alleviated markedly and even the tumor may be regressed completely after acute infection. Bacterin OK-432 has notable inhibitory effect on the growth of various tumors in animals. At present, OK-432 has been used in clinical immunotherapy for tumors with no other adverse events besides fever and leucocytosis. This study was to investigate the effects of combined bacterin on the serum level of interleukin 12(p70)[IL-12(p70)] and the growth of sarcoma 180 (S180) in mice. METHODS After transplantation of S180, the mice were randomized into 5 groups, and received injection of Staphylococcus aureus, Salmonella typhimurium, Mycobacteria phlei, and combined bacterin containing the 3 bacteria strains, respectively, or received no treatment (blank control). The weight of S180 xenografts, the thymus, and the spleen in mice was measured. The serum level of IL-12(p70) was detected by ELISA. RESULTS The mean weight of S180 tumors was 1.39 g in Staphylococcus aureus group, 1.50 g in Salmonella typhimurium group, 1.36 g in Mycobacteria phlei group, 0.62 g in combined bacterin group, and 2.40 g in blank control group; the differences among the 5 groups were significant (F=66.73, P<0.001). The mean weight of S180 tumors was significantly lower in the 4 bacterin groups than in control group, and significantly lower in combined bacterin group than in the 3 single bacterin groups (q test, P<0.001). The weights of the thymus and the spleen among the 5 groups had no significant difference (F=2.36, P>0.05; F=1.89, P>0.05). The inhibition rate of tumor growth was significantly higher in combined bacterin group than in Staphylococcus aureus, Salmonella typhimurium, and Mycobacteria phlei groups (74.17% vs. 42.08%, 37.50%, and 43.33%, P<0.01). The mean serum level of IL-12(p70) was 19.44 pg/ml in combined bacterin group, 12.41 pg/ml in Staphylococcus aureus group, 10.35 pg/ml in Salmonella typhimurium group, 11.68 pg/ml in Mycobacteria phlei group, and 4.45 pg/ml in control group; the difference among the 5 groups was significant (F=15.76, P<0.0001), but the difference among the 3 single bacterin groups was not significant (q test, P>0.05), while the differences between other groups were significant (q test, P<0.01). CONCLUSIONS The chosen bacterins in this study can induce the mice to produce IL-12(p70) and suppress the growth of S180. The effect of the combined bacterin is much better than the single bacterins.