Until and including 1987 diagnostically significant serological titres to swine brucellosis had occurred in the serum agglutination test (SAT) or the complement fixation test (CFT), ie, > or = 100 iu or > or = 20 icftu, respectively, almost every year since reliable records began, but usually only about 0.05 per cent in the SAT and 0.005 per cent in the CFT. Brucella suis was never isolated by cultural examination. In 1988 the level of CFT reactions > or = 20 icftu rose to 0.42 per cent (1.04 per cent in the last quarter of the year) but the SAT reactions remained relatively unchanged. In 1989 the levels of both CFT and SAT reactions increased further with CFT reactions again predominating. Analyses of the serological reaction patterns in individual herds suggested that infection with brucella or some other organism capable of causing serological cross-reactions had become widespread in Great Britain, although signs of disease typical of swine brucellosis had not been observed. Some herds had reactions which persisted for many months whereas others showed them for only a short time. In early 1990 Yersinia enterocolitica serogroup O:9 was isolated from some pigs purchased from one of the reactor herds and this organism is probably responsible for the increased numbers of seroreactions. It had not previously been found in pigs in Great Britain.