Walker carcinosarcoma cells were cultured as an ascitic tumor in the rat. These cells were studied for their response to human chemotactic factors derived from complement. Complement activation for tumor chemotaxis was performed by reaction of serum with a tumor extract. An anticomplementary agent, K-76 sodium monocarboxylic acid (K-76 COONa), was tested in these systems. It was found to be effective in blocking the formation of a chemotactic factor for tumor cells from human complement at a concentration of 300 micrograms/ml. Addition of K-76 COONa after complement activation had no effect. No effect on random migration of tumor cells was found at concentrations of 500 micrograms/ml. No effect on tumor cell viability was found up to 500 micrograms/ml. At 1000 micrograms/ml there was a 15% decrease in viability (p less than 0.05). Since chemotactic mechanisms (probably from activated complement) may play a role in the movement of tumor cells, this apparently low toxic anticomplementary agent (K-76 COONa) may be of value in the prevention of tumor metastases.