The protective efficacy of a recombinant Taenia ovis vaccine antigen, 45W, was compared in sheep vaccine trials with antigen expressed by the full length 45W cDNA and by incomplete copies of the cDNA. Vaccine, trials were also carried out using antigen expressed by a cDNA (45S) having a sequence similar, but not identical, to 45W. Stability of the 45W antigen expressed in Escherichia coli was found to be increased after deletion of cDNA sequence encoding 19 COOH-terminal amino acids. This truncated form of the antigen was designated 45WB/X. Vaccination of sheep with antigen expressed by 45W, 45WB/X, as well as full length 45W and 45S cDNAs, induced high levels of protection. Vaccination with antigen expressed by an incomplete copy of the 45S cDNA clone did not induce protection. Comparison of deduced amino acid sequences for these clones suggests that the host-protective epitope(s) of the 45W antigen occur on either or both of the 23 and 9 amino acid peptides at the amino and carboxyl termini of 45W, respectively. Antibody binding epitopes of 45W were investigated in ELISA using overlapping 9 amino acid peptides. Protection was found to correlate with the induction of antibody to two 9 amino acid peptides.