Meanwhile, a physician-scientist through and through, McCarty turned his attention to diseases promoted by streptococci. So it happened that on the retirement of Homer Swift in 1946, McCarty was asked to head the laboratory established in 1922 to work on streptococci and rheumatic fever. This was the scientific home of Rebecca Lancefield, who developed the still powerful serological classification of streptococci. From innumerable clinical observations, combined with Lancefield's classification, it was clear that acute rheumatic fever, a severe sterile inflammatory condition affecting particularly the joints and the heart, was a complication of group A streptococcal pharyngitis, following the infection by several weeks. The causal chain of events still eludes us. McCarty attacked this problem by studying both the biology of group A streptococci and patients with acute rheumatic fever admitted to the Rockefeller Hospital.