Nonobstructive pyelonephritis was produced in the rhesus monkey (Macaca mulatta) by means of retrograde inoculation of Escherichia coli to the point of pyelotubular backflow. The effects of treatment with cyclophosphamide or azathioprine were determined. Commensal renal viruses were not activated by the immunosuppression, and thus did not complicate our results. Both cyclophosphamide and azathioprine prolonged the bacteriuria and produced more severe pathology. Cyclophosphamide decreased the leukocytic response and partically suppressed the antibody response. The increased amount of acute pyelonecytic response and partially suppressed the antibody response. The increased amount of acute pyelonephritic lesions after treatment with this drug suggest that the antibody and inflammatory responses may be important protective mechanisms, particularly regarding prevention of abscesses. In contrast, azathioprine did not decrease the leukocytosis nor the antibody responses, but resulted in decreased in vitro responsiveness of lymphocytes to mitogens. The increased severity of chronic pyelonephritic lesions after azathioprine treatment suggests that the cellular immune response also may be an important protective mechanism during late stages of the disease. The results thus indicate that the immune response is protective and is not directly responsible for the chronic scarring of pyelonephritis.