Two Actinobacillus pleuropneumoniae serotype 7 antigens were expressed in Escherichia coli and tested for their protective efficacy in an experimental pig model. The antigens used were a fusion protein containing the carboxy-terminal 70% of the +/- 103 kDa cytolysin and a full length 60 kDa protein which has been shown previously to bind transferrin. Pigs were immunized twice with 25 micrograms of either or both preparations. All pigs developed a strong humoral immune response comparable to that induced by infection. Upon challenge with an A. pleuropneumoniae serotype 7 strain, all immunized groups were less affected by the disease and showed significantly lower mortality than the controls (p less than 0.1). Pigs receiving both antigens had a tendency to recover faster than the controls or animals which were vaccinated with only one antigen. Protection was serotype-specific since no cross-protection was detected following challenge with an A. pleuropneumoniae serotype 1 strain. A dose-response experiment using the single antigens at 200, 50 or 12.5 micrograms per dose showed no difference in protection among the groups.