Antibody production in humans and three species of laboratory animals infected with Rickettsia rickettsii was determined with the indirect hemagglutination test. Rabbits, guinea pigs, and mice were inoculated with R. rickettsii and bled at intervals. Antibody which agglutinated both fresh and glutaraldehyde-fixed sheep erythrocytes sensitized with antigen prepared either from purified rickettsiae or from infected yolk sacs was found in rabbit sera at all intervals tested (10 to 59 days postinfection). Antibody which agglutinated fresh but not glutaraldehyde-fixed erythrocytes sensitized with either of the above antigens was detected in guinea pig sera obtained 7, 14, and 28 days postinfection. Antibody was found in mice inoculated with 5.6 x 10(6) plaque-forming units of R. rickettsii but not in mice given 5.6 x 10(2) plaque-forming units. Peak indirect hemagglutination titers occurred in nonvaccinated human Rocky Mountain spotted fever patients about 3 weeks after onset of illness, and antibody was still detectable after 1 year. Both human immunoglobulin G and human immunoglobulin M antibodies agglutinated sensitized cells, but immunoglobulin M antibodies apparently were more efficient. The indirect hemagglutination test is useful for the titration of human, rabbit, guinea pig, and mouse antibodies when the appropriate erythrocytes are used.