Concanavalin A (Con A) and trypsin inhibitor isolated from Acacia confusa were covalently linked with N-succinimidyl-3-(2-pyridyldithio)propionate. Con A-A. confusa trypsin inhibitor (ACTI) conjugate covalently bound (Con A-ACTI) retained about 42% of the trypsin inhibitory activity present in the native ACTI and had a higher hemagglutinating activity than did the native Con A. Con A-ACTI had a greater resistance to tryptic digestion than did the mixture of Con A and ACTI. The conjugate entered sarcoma 180 tumor cells, whereas the free ACTI did not. A single dose of the conjugate injected ip into noninbred N:NIH(S) white mice bearing sarcoma 180 had a remarkable effect of increasing the survival of tumor-bearing mice, while the mixture of an equivalent dose of free Con A and ACTI was not effective.