Augmentation of antitumor activity of antitumor drugs in combination with Lactobacillus casei HY2782 (LC2782) was studied against Sarcoma-180 (S-180) and Lewis lung carcinoma (3LL). Antitumor drugs used in this study were 5-fluorouracil (5-fu) and cyclophosphamide (CP). The prolongation effect of LC2872 on the life span of mouse intraperitoneally implanted with S-180 was stronger than that of OK-432 and BCG, while the inhibitory effect of OK-432 and BCG on the growth of 3LL solid tumor was a little stronger than that of LC2782. Average survival rates of mice administrated LC2782, OK-432 and BCG were 192%, 141%, and 112%, respectively, when that of the control was 100%, Intralesional administration of 5-Fu, CP, 5-Fu+LC2782 and CP+LC2782 resulted in 93%, 69%, 99% and 73% inhibition rates against 3LL solid tumor proliferation. The combination therapy of 5-Fu or CP with LC2782 significantly prolonged the life span of S-180-inoculated ICR mice. Average survival rates of mice administrated 5-Fu and CP alone were 115% and 99%. Furthermore, survival rates of mice administrated 5-Fu and CP in combination with LC2782 were 226% and 244%, respectively.