Tillmann et al., 1997: "Subcutaneous soft tissue tumours at the site of implanted microchips in mice." Experimental and Toxicologic Pathology. 1997;49:197--200.
Summary: 4,279 mice were injected with microchip implants for identification purposes as part of a larger study. Of these, 36 developed malignant tumors (fibrosarcoma and malignant fibrous histiocytoma) that were "clearly due to the implanted microchips." Control animals as well as experimental animals developed the tumors.
Excerpts: "The neoplasms induced in the present investigation are clearly due to the implanted microchips." // "Further information on [tumors] induced by microchips, e.g., experiments on their chemical components (glass and polypropylene cap), or the physical presence of the implant alone are necessary."