Summary To study the migration of Leptospira interrogans serotype pomona through the kidney, conventionally-reared mice aged 2 or 3 weeks were infected intraperitoneally with this organism. Within the first 4 days, the organisms migrated from the capillary lumina to the interstitial tissue and provoked an interstitial oedema. By the 10th day they were seen between the epithelial cells of the proximal convoluted tubules and by the 14th day many were located within tubular lumina. There was no evidence of viable leptospires within the cells of the proximal tubules, though occasionally structures resembling leptospiral fragments inside lysosomes were observed. At no stage during the study were glomerular lesions seen.