A recently described latex agglutination test was used to determine peptidoglycan antibody titers in sera from healthy human subjects and in sera from patients with a history of streptococcal infections or rheumatoid arthritis. Using latex particles coated with group A streptococcal peptidoglycan 32.8% of the sera from 961 healthy donors reacted positively with titers ranging from 1 : 5 to 1 : 320. Peptidoglycan antibodies were more frequently present in the younger population (45.1% in the 20-29 years old) and considerably decreased in advanced age (15.7% in the 70 years or older). Sera from 82 patients with elevated ASO-titers showed detectable peptidoglycan antibody levels in 40.2%; a statistically significant correlation between ASO and peptidoglycan antibody titers could not be substantiated. Sera from 25 patients with rheumatoid arthritis gave a high incidence (56-92%) of positive results. However there was evidence that this may be due to the action of rheumatoid factor present in such sera.