In this study the induction of the acrosome reaction of canine sperm by homologous zona pellucida (ZP) was examined. Twelve semen samples obtained from 6 normal beagle dogs were evaluated after sperm incubation in vitro with canine capacitation medium (CCM). Washed sperm were preincubated at 37 degrees C in 5% CO2 in air for 4 and 7 h prior to experimental treatment. Sperm were co-incubated for 1 min with intact oocytes collected from canine ovaries. Half of the oocytes were then fixed, and the bound sperm were assessed for acrosome reactions through use of a polyclonal antisperm antiserum and indirect immunofluorescence. The remaining oocytes were incubated in sperm-free medium for an additional 1-h period, and the acrosomal status of sperm bound to the ZP was evaluated similarly. The percentage of acrosome-reacted sperm on the ZP increased significantly during the 1-h incubation period. In other experiments, capacitated canine sperm were incubated with heat-solubilized ZP for 1 h and their acrosomal status was determined using fluoresceinated Pisum sativum lectin. The percentages of acrosome-reacted sperm increased significantly in ZP solution compared with controls. These data demonstrate that intact and solubilized canine ZP are capable of inducing acrosome reactions of canine sperm.