In vitro chemosensitivity was evaluated by succinate dehydrogenase inhibition (SDI) test in 94 human tumors including 59 gastric cancers, 27 colo-rectal cancers and 8 malignant lymphomas. Tumor fragments were exposed to 12 kinds of antitumor drugs at ten times peak plasma concentration. Evaluable rates were 86/94 (91%) for all cases, 56/59 (95%) for gastric cancers, 22/27 (81%) for colo-rectal cancers and 8/8 (100%) for malignant lymphomas. The mean of SD activity was decreased to 48% of that of control cells with aclacinomycin, 49% with carboquone, 53% with actinomycin D, 54% with mitomycin C and 54% with daunomycin for gastric cancers, 59% with adriamycin for colo-rectal cancers and 33% with cyclophosphamide (40487 S), and 33% with actinomycin D, 37% with vinblastine and 39% with adriamycin for malignant lymphomas. When the SD activity was reduced to below 50% by antitumor drugs, the chemosensitivity was defined as positive. The antitumor drugs which had a higher chemosensitive-positive rate were aclacinomycin, carboquone and mitomycin C for gastric cancers, adriamycin for colo-rectal cancers and 40487 S, daunomycin and vinblastine for malignant lymphomas. Our results suggest that the origin of a tumor is a critical factor in its chemosensitivity.