Injectable bovine collagen has found extensive applications as soft tissue substitute and, more recently, in the primary treatment of vesico-ureteral reflux. Reactions to the material have been reported to be temporary, of low incidence and self-limiting. However, animal collagen is still an immunogen that is capable of eliciting an immune response. Elevated anti-collagen antibody levels have been detected in patients treated with clinical doses of injectable collagen, even in the absence of adverse cutaneous reactions. These antibodies have not yet been shown to be cross-reactive with human collagen and it is presumable that for most patients biologic exposure to collagen does not pose any health hazard. Nevertheless, the long-term significance of these antibodies is unknown and, until the nature of the immune response to bovine collagen is better understood, it would be advisable to avoid collagen implants in patients with a long life-expectancy or when other therapeutic modalities are available.