Administration of carrageenan to tuberculin-sensitive guinea pigs resulted in a dose-dependent suppression of delayed hypersensitivity. Recovery was complete 1 week after drug administration. The suppressive activity was not correlated with anticomplementary or anticoagulant activity. It is suggested that carrageenan exerts its effect as a result of its toxicity for macrophages, acting through lysosomal damage and the release of intracellular enzymes.