Ball et al., 1991: "Evaluation of a microchip implant system used for animal identification in rats." Laboratory Animal Science. 1991;41(2):185—186.

Summary: 40 rats were implanted with subcutaneous microchips. The tissue surrounding the implants was evaluated for adverse reactions after periods ranging from two weeks to one year. (See table below.) No palpable masses or cancerous changes were observed.

Author(s)	Species	# Animals	Length of Microchip Exposure	Developed Cancer
Ball et al., 1991	rats	10	2 weeks	none observed
		10	3 months
		10	6 months
		10	1 year

Note: When Elcock et al. (2001) later studied a larger sample of rats (n = 1,040) over a two-year period, they found a nearly 1% incidence of malignant tumors.