From the Departments ofPathology, the Lancaster Moor Hospital, Lancaster, and the *Manchester Medical School, Manchester suMMARY A polyclonal antiserum to toxic shock syndrome toxin (TSST-1) and a standard immunoperoxidase technique were used on formalin fixed tissues from 50 cases of sudden infant death syndrome (SIDS) to determine if the syndrome was associated with bacterial infection. There was strong specific staining in the renal tubular cells in nine (18%) cases. A similar pattern of staining was seen in three ofa series of50 kidneys selected for comparison from a wide range ofnecropsy cases. The staining was finely granular within the cytoplasm of proximal convoluted tubular cells and diffuse in tubular cell nuclei. In an attempt to validate the staining pattern the immunoperoxidase technique was also performed on formalin fixed kidneys from rats which had been given intravenous injections ofcrude bacterial products containing TSST-1. These showed coarse granular cytoplasmic staining in proximal convoluted tubules with some diffuse nuclear staining. This pattern was not seen in controls injected with saline. These results indicate that TSST-1 might have a pathogenic role in some cases of SIDS.