Three cases of nephritis after mild upper-respiratory-tract infection occurred in five members of a family running a small dairy farm in North Yorkshire, Streptococcus zooepidemicus (Lancefield group C) was cultured from the throats of three of those affected. The organisms were shown to produce endostreptosin, a recently discovered cytoplasmic antigen of certain beta-haemolytic streptococci that has been shown to be of prime importance in the development of poststreptococcal glomerulonephritis (PSGN). Serological testing of the patients showed raised and persistent antibody tires to this substance. These findings substantiate the association, suggested by the epidemiological features of a similar outbreak that occurred in Rumania in 1968, between Strep zooepidemicus and nephritis. As in several previous reports of Strep zooepidemicus in man, infection appeared to have been acquired by the consumption of unpasteurized milk. This outbreak implicates non-group-A streptococci in the aetiology of PSGN.