The studies reported here were designed to examine the effects of intratumoral preoperative administration of Bacillus Calmette-Guerin (BCG) on the cure rates of C3H mice transplanted with MH134 tumor cells and on the metastatic rates in the regional lymph nodes. Furthermore, the morphological findings occurring in the regional lymph nodes were monitored during tumor growth using H-E stain and non-specific esterase staining. The cure rate of the Group treated with BCG intratumoral injection and surgery was significantly higher than that of the Group treated with surgery alone, and in the BCG + surgery group metastatic rates of regional lymph nodes decreased consistently after operation. Moreover, in this group, extensive sinus histiocytosis and marked swelling of the regional nodes were frequently observed. Quantitative studies of the cell kinds using the esterase staining indicated that intratumoral injection of BCG has an effect on the influx of lymphoid cells into the regional nodes, but does not aid specific cell lineage to flow into the regional nodes. In cytostatic assays, it was shown that the regional lymph node cells and spleen cells in the BCG + surgery group always have a greater per cent of inhibition than those in the surgery alone group.