Rao & Edmondson, 1990: "Tissue reaction to an implantable identification device in mice." Toxicologic Pathology. 1990;18(3):412–416.

Summary: 140 mice were implanted with subcutaneous microchips and evaluated for adverse reactions after periods ranging from three months to two years. No neoplastic (abnormal tissue) reactions were observed.

Author(s)	Species	# Animals	Length of Microchip Exposure	Developed Cancer
Rao & Edmondson, 1990	mice	10	3 months	none observed
		10	15 months
		74	2 years
		39	Less than 2 years

Note: When Le Calvez et al. (2006) later studied a larger sample (n=1,260) of the same breed of mouse, they found a 4.1% incidence of microchip-induced tumors.