Casts with numerous and unusually large granules were seen in the urine of a child with renal Fanconi's syndrome. When the urine sediment was sealed under a coverslip for several days, many granules changed to filamentous bacterial variants that segmented and, finally, appeared as streptococcal-like forms. When the patient's blood was cultured by a special method, bacterial variants grew consistently, and frequently reverted to parent coccal forms, although conventional cultures were negative. Variants from blood cultures had the same morphology and staining properties as granules in casts and in cystic structures found within hypertrophied renal pelvic epithelial cells. Cryptic parasitization with bacterial variants probably occurs in many nephropathies. Variants are known to produce toxins and immunogens, which could lead to mesangial and basement membrane deposits as well as to occlusive reactions in the renal microcirculation.