The evaluation of the immunotoxicity of medical devices as part of their biocompatibility assessment is the subject of the most recent ISO-10993 guidance (part 20). Traditionally, and still the case for most skin only contact devices, delayed contact, simple dermal sensitization was adequate for evaluating this endpoint. This is in spite of the fact that the association between implanted or indwelling devices and granuloma formation has been known for some time (Adams, 1953; Anderson, 1988; Black, 1981; Burkett et al., 1986; Woodward and Salthouse, 1986; Unanue, 1994; Salthouse, 1982; Marchant et al., 1985). However, improved science and the record of immune based device problems since 1980 has brought the adequacy of this approach into question.