Twenty-six free-roaming feral mares were immunized against porcine zonae pellucidae (PZP) between February and May, 1988. Eight sexually mature mares received 2 inoculations 2 weeks apart, and 18 mares received 3 inoculations at intervals of 2 and 4 weeks. Analysis of urinary oestrone conjugates (E1C) and non-specific progesterone metabolites (iPdG) in samples collected in October, 1988, revealed that none of the 18 mares that received 3 and only 1 of the 6 mares that received two inoculations were pregnant, whereas 3 of 6 sham-injected control mares and 5 of 11 untreated mares were pregnant. In February and March, 1989, 14 of the immunized mares that were not pregnant were given a single booster inoculation of PZP. Urinary E1C and iPdG analysis, from samples collected in August and October, 1989, revealed only a single pregnancy among the 14 boosted mares, whereas 33% of mares treated in 1988, but not given a booster inoculation in 1989, and 7 of 16 untreated controls, were pregnant. Foaling was reflected accurately by the urinary steroid metabolite analyses. These results suggest that once recognition of PZP antigen occurs in mares, subsequent annual inoculations will raise antibody titres sufficiently to inhibit fertility.