The complete sequence of the cDNA encoding the nematode polyprotein allergen/antigen (NPA) of the bovine lungworm Dictyocaulus viviparus was obtained by immunoscreening of cDNA expression libraries and by 5' RACE (rapid amplification of cDNA ends). The encoded polypeptide is similar in sequence to the ABA-1 allergen of Ascaris, the gp15/400 'ladder' protein of Brugia malayi, Brugia pahangi and Wuchereria bancrofti, and a 15-kDa antigen of Dirofilaria immitis. As with these, the predicted amino-acid sequence comprises a head-to-tail array of similar polypeptides with regularly spaced consensus proteinase cleavage sites. The D. viviparus protein was designated DvA-1 (D. viviparus antigen-1) and the gene dva-1. The deduced amino-acid sequence of DvA-1 showed features not observed before in other NPAs: (i) a hydrophobic leader peptide is present, (ii) none of the 12 units in the array are identical and the sequences diverge to a degree hitherto unseen in the NPAs of other nematode parasites, (iii) the predicted proteinase cleavage sites are also diverse in sequence and, in two instances, no consensus cleavage site was identifiable at the expected position, (iv) a short repeat unit is present, which is the only one containing a consensus N-glycosylation site and (v) a C-terminal extension peptide is encoded which shows no similarity to that from A. suum ABA-1. Comparison of independent cDNAs revealed slight variations in the sequence of the gene within the parasite population. Antisera to recombinant DvA-1 polypeptide identified 14-15-kDa antigens in both parasite somatic and excretory-secretory material. DvA-1 is the only NPA for which the complete coding sequence is available and the new principles which it illustrates may lie unsuspected in the NPA-encoding genes of all nematode parasites.