Whereas in vitro assays of antibody responses to infectious agents are commonly used for routine diagnostic purposes, diagnostic tests for cellular responsiveness are confined to in vivo intradermal tests. This paper describes a simple and rapid in vitro cellular assay for bovine tuberculosis. The assay system is based on the detection of gamma interferon (gamma IFN), which is released in response to specific antigen. This assay can be carried out with whole blood samples thus avoiding the time-consuming task of isolating lymphocytes. A simple bioassay has been developed to quantitate the amount of gamma IFN produced, but this will eventually be replaced with an enzyme-linked immunoassay using monoclonal antibodies specific for bovine gamma IFN. The application of this system to bovine tuberculosis and other infectious diseases may provide a convenient in vitro cellular assay for routine diagnostic purposes.