Confirming Parelaphostrongylus tenuis infection in moose (Alces alces) and other susceptible hosts is difficult. An enzyme-linked immunosorbent assay (ELISA) was developed using the excretory-secretory (ES) products of third-stage P. tenuis larvae (ES-ELISA) and the test applied to serum samples obtained from seven moose calves (5-9.5 mo old) given infective larvae (L3) in doses approximating those likely to be received in nature (3-30 L3). Anti-P. tenuis immunoglobulin G antibodies were detected in all seven inoculated moose during the course of infection until the termination of experiment 61-243 days post-inoculation (DPI). Five animals tested between 16-25 DPI had significant antibody levels, while a sixth animal did not test positive until 46 DPI. The seventh animal was not tested until 199 DPI. Antibody levels remained elevated in all five animals that harbored adult worms at the termination of the experiment. Whereas, antibody levels showed a gradual decline in the two remaining animals, presumably because of death of worms, and antibodies were undetected in one animal at the time of necropsy. The other animal displayed an anamnestic increase in antibody level following a challenge inoculation of infective larvae. Terminal and peak optical density (OD) values detected by ES-ELISA strongly correlated with inoculation dose (r = 0.98, P = 0.02 and r = 0.95, P = 0.04, respectively) among animals harboring adult worms (n = 4) but not significantly with the number of worms recovered postmortem (peak OD, r = 0.82, P = 0.18; terminal OD, r = 0.93, P = 0.07). Unlike the ES products, use of somatic antigens of the adult worm in ELISA did not provide satisfactory results. Antibodies to P. tenuis were detectable by ES-ELISA in two of 21 free-ranging moose from an enzootic area but not from any of 23 animals from a non-enzootic area. The ES-ELISA appears to be a useful test for assessing exposure of moose to P. tenuis.