Antibody-dependent cell-mediated cytotoxicity (ADCC) was studied using as targets 51Cr-labelled Vero cells infected with the Bartha strain of Aujeszky's disease virus (ADV). Using hyperimmune anti-ADV serum to sensitize the targets, porcine leukocytes from dextran-sedimented blood were found to be efficient effector cells yielding maximal 51Cr release by 16 hours. Whilst complement-dependent cytotoxic antibody could be demonstrated no enhancement of ADCC by complement was found. The sera of pigs vaccinated i.m. with Bartha virus were titrated in ADCC using leukocytes as effector cells and the results compared with those obtained by virus neutralization. ADCC proved to be a much more sensitive technique and might, therefore, provide the basis for a reliable diagnostic test. Partially purified lymphocytes and polymorphonuclear leukocytes from blood and peritoneal exudates, and macrophages from exudates were found to mediate ADCC with hyperimmune serum, but differences were observed in the efficiency and timing of their cytotoxic effects.