Immunoblotting of urine from 21 patients of both sexes and of wide age range who had a Proteus mirabilis urinary tract infection (UTI) showed that 14 (64%) specimens contained immunoglobulin A (IgA). In nine (64%) of these the IgA heavy chain had been degraded to fragments of a size identical to those formed when purified IgA was degraded by pure P. mirabilis protease. Urine from patients with clinical evidence of upper UTI contained fragmented IgA and in some of these urine samples P. mirabilis protease activity was detectable. Urine infected with a non-proteolytic strain contained only intact IgA. It is concluded that P. mirabilis IgA protease is produced and is active during infections of the urinary tract.