Ten monoclonal antibodies (MAbs) raised against porcine zonae pellucidae (PZP) were checked for their reactivity against zonae of other species. Two (MAbs A6 and C10) out of six MAbs checked reacted with canine zonae as ascertained by indirect immunofluorescence. One of these cross-reactive MAbs (A6) inhibited the penetration of canine ZP by canine spermatozoa in an in vitro assay by 62.5% and 22% at 10 micrograms/ml and 1 microgram/ml, concentration, respectively. Epitopes recognized by MAbs A6 and C10 are present on the 55-kDa glycoprotein of PZP as determined by Western blotting experiments. None of the ten anti-PZP MAbs reacted with zonae of the bonnet monkey, hamster or rabbit. Of the six MAbs checked, none showed cross-reactivity against mouse and cat zonae, and of four checked, none reacted with guinea pig zonae. Only one of the ten showed slight reactivity with human zonae.