FREQUENCY OF SPONTANEOUS TUMORS IN NMRI MICE IN 21-MONTH STUDIES

Tumours occurring in 900 male and 900 female mice of the strain NMRI ( BOR: NMRI (SPF Han), breeder: Winkelmann, Borchen) employed as control groups in 18 different studies lasting for 21 months on average (stud y No. 8 for 20 months, study No. 5 for 24 months) were compiled on the basis of histopathology reports. 883 male and 874 female mice were ev aluated. The remaining animals were unavailable due to autolysis. A to tal of 1522 tumours (689 in males, 833 in females) were seen in 1061 m ice (493 males, 568 females), 830 of which were classified as malignan t (318 in males, 512 in females). About 70% of all primary tumours wer e located in the haemolymphoreticular tissue, lungs, ovaries, adrenals and uterus. Average incidences of primary tumours found in the follow ing organs were: haemolymphoreticular tissue 22.9%, lungs 21.6%, ovari es 12.1%, adrenals 6.9%, uterus 6.6%, liver 4.4%, Harderian gland 4.4% , pituitary 3.5%, mamma 3.3% and various other organs with a tumour fr equency below 3%. Male mice exhibited markedly more tumours of the lun gs, adrenals, liver and Harderian glands, while tumours of the haemoly mphoreticular tissue and pituitary were predominant in females. A comp arison of these data with those from preceding two-year or life-span s tudies using NMRI mice from other breeders revealed a certain increase in primary tumours of the adrenals, Harderian glands as well as pitui tary. This result could. be due to more frequent examinations.