Immunocontraception using porcine zona pellucida (PZP) vaccines is being explored as a nonlethal method of solving the problems of locally overabundant wildlife populations. This study characterized the immunological response of captive elk (Cervus elaphus nelsoni) to PZP challenge using 18 3-yr-old cows and was conducted from 14 September 1994 to 13 December 1995. All animals were given a single PZP inoculation and 1 mo later six of these animals were randomly chosen and received a booster inoculation. Blood samples were drawn from all animals at the time of the initial inoculation and 1, 2, 4, 6, 10, and 15 mo later. Immunological response was assessed by measuring anti-PZP antibody levels in serum. All animals demonstrated a strong immune response with no evidence that the booster enhanced antibody levels. Antibody levels rose from between 0 and 4 at the time of the initial injection to peak levels of 85 to 163 within 2 to 6 mo, followed by a noticeable decline by 15 mo post-vaccination. Limited data suggest that antibody levels >100 may be required to effect contraception. High individual variability in immune response observed in this study suggests it may be difficult to predict the proportion of animals effectively treated. Disruption of seasonal synchrony in calving also could occur if antibody levels in individuals fall below effective levels while animals are still cycling. These results indicate that immunocontraception using PZP vaccines is possible for elk. However, carefully controlled population experiments will be required in order to assess the potential and limitations for management applications of this technique.