Pyelonephritis is accompanied by complex immunological reactions, the study of which is just now being undertaken. Study of 216 patients with a significant monomicrobial urinary infection showed that in more than 95 per cent of cases of Gram negative infection the urine contained IgG type antibodies, often associated with IgA. These antibodies may be easily detected using an immunofluorescent technique, which would appear to be the best method available at present for distinguishing between pyelonephritis and an infection of the lower urinary tract. They may also be found in the serum, though less frequently. Detection by immunofluorescence is markedly more sensitive than using the classical passive haemagglutination technique. The biological significance of these antibodies remains imprecise, and their protective or facilitating role may be discussed. Their specificity must be determined, in order that sequential study of these immunological reactions may be of help in the conduct of treatment.