Helyer and Howie1 described a disease of NZB/BL × NZY/BL and NZB/BL × NZW/BL F1 hybrid mice closely resembling human systemic lupus erythematosus (SLE) in renal histology. A breeding nucleus of these mice was provided by Drs. F. and M. Bielschowsky and Dr. Howie, to allow confirmation of their histological findings. Similar lesions were found in NZB/BL mice, although the age of onset is later than in the B × W hybrids. Outbred albino Palmerston North (PN) mice, initially intended as controls, showed renal and arterial lesions more closely resembling human polyarteritis nodosa than SLE. A comparison of the two types of lesion is summarized in Table 1. Spontaneous arteritic lesions have been described in rats by Skold2 and in mink by Leader3. Deringer4 has described arteritis of the ovarian and small bowel arteries in BL/De mice, but generalized spontaneous arteritis in mice does not appear to have been described previously.