The protein profile and antigenic properties of lung-stage larvae of Ascaris lumbricoides and A. suum were studied using 2-dimensional electrophoresis and immunoblot analysis, respectively. The protein profiles of the 2 parasites were identical except for the presence of only 1 major protein spot specific for each. There was a complete cross-reactivity between the 2 parasites at the immunological level, and no specific antigen was recognized using specific antibody raised against the 2 parasites in rabbits.