In parallel, McCarty studied patients with rheumatic fever admitted to the Rockefeller Hospital as well as valuable specimen collections from military outbreaks of the disease during World War II. He and his collaborators found that antibody responses to several streptococcal antigens were significantly higher in the group of individuals that developed acute rheumatic fever than in individuals with uncomplicated infection. However, the response to unrelated antigens, for instance, diphtheria toxoid, was not enhanced. He found that group A streptococci secreted unusually high amounts of DNase, and established a test for the detection of antibodies produced in response to this antigen. This led to the discovery that streptococci were able to produce multiple isozymes of DNase. He purified human C-reactive protein through crystallization, produced a highly specific antiserum, and, using this much simpler and more sensitive test, found that C-reactive protein levels responded more rapidly and reliably than other inflammatory markers and could serve as the most accurate indicator of rheumatic inflammatory activity. Measuring C-reactive protein levels to detect inflammation is routine now in medical practice.