Soluble antigen prepared from pseudorabies virus-infected embryonic pig kidney cells was used to demonstrate the presence of pseudorabies virus antibodies in sera taken from naturally and experimentally infected swine. Antibody could be detected by agar-gel immunodiffusion as early as 14 days postexposure and was demonstrable at least 3 months after experimental infection (the longest period tested). The reliability of the agar-gel immunodiffusion test was comparable to that of the microtitration serum-virus neutralization test commonly used in diagnostic laboratories. One advantage of the agar-gel immunodiffusion test was that severely hemolyzed and cytotoxic serum samples could be tested with confidence. The test is simple, rapid, and inexpensive and could be easily adopted by diagnostic laboratories that receive requests for pseudorabies virus antibody determinations in swine sera.