Elcock et al., 2001: "Tumors in long-term rat studies associated with microchip animal identification devices." Experimental and Toxicologic Pathology. 2001;52:483--491.
Summary: 1,040 rats were implanted with microchip implants for identification purposes as part of a larger study. After two years, just under 1% had developed malignant tumors surrounding the implants. The researchers attributed the tumors to the presence of the microchip, and referred to them as "microchip-induced."
Excerpts: "Electronic microchip technology as a means of animal identification may affect animal moribundity and mortality [i.e., illness and death rates], due to the large size and rapid growth of microchip-induced tumors as well as the occurrence of metastases." // "Most tumors arising from foreign bodies are malignant . . . and have a rapid growth rate, killing the animal in a matter of weeks."