Table 2.

Spontaneous tumors in germ-free (GF) experimental rats and mice.

GF Spontaneous	Incidence %	Main Types of Tumors	Strain	Gender	n. of Animals	Time of Observation	Year	Citation
Rats	25/? *	mamary gland, thymus	wistar	♀, ♂	359	up to 38 months	1963	[28]
	0 **	-	fisher
	0 **	-	sprague-dawley
	0%	-	wistar	♀, ♂	33	older than 24 months	1970	[29]
	13%	prostate	wistar	♂	31	20–40 months	1973	30
	19.6% ♀	mamary gland	F344	♀	78♂	from 7 months to >3 years	1976	[12]
	11.5% ♂			♂
	36% ♀	leukaemia		♀	102♀
	25.6% ♂			♂
Mice	30%	mamary gland	C3H/Pi	♀	66	up to 24 months	1967	[31]
	21%	lung	BALB/cPi	♀, ♂	67 (15♂, 52♀)	median age 21 months	1971	[32]
	3%	mamary gland
	4.5%	lympho-sarcoma
	3%	reticulum cell sarcoma
	3%	skin carcinoma
	3%	adrenal cortical carcinoma
	1.5%	ovaries		♀
	14/? *	leukaemia	AK	ns	ns	mean age 8,7 months	1965	[33]
	68%	small bowell—polyposis	BALB/c	♀	40	12 months	1984	[34]
	89%			♂	36
	39%	liver	C3H/He	♂	49	11 months (48 weeks)	1979	[35]

(*) unknown number of total Wistar rats, here only the absolute number of tumor bearing animals; (**) all animals negative for tumor in the observed strain.