Mice treated with tetracycline have reduced capacity to mount delayed-type hypersensitivity (DHT) responses to sheep erythrocytes. The suppressive effect of DTH was more pronounced when the drug was given at the time of challenge with sheep erythrocytes. The in vivo chemotaxis of macrophages was also depressed in mice given tetracycline. There was, however, no significant suppression of antibody production to sheep erythrocytes.