Evaluation of diagnostic sensitivity and specificity was based on test results of 346 sera from pigs known to be infected and 139 sera from pigs known not to be infected. All sera were tested with a monospecific antigen (serotype 1) and a polyspecific antigen (serotypes 1-5). The sensitivity of the polyspecific antigen was approximately 85% at serum dilution 1:2 and was significantly higher than the monospecific antigen at all serum dilution levels. The specificity of the two antigen preparations was not significantly different at any dilution and increased from approximately 78% to 1:2 to 100% at 1:128. When pigs from herds with unknown incidence of infection were studied, it was found that a high proportion seroconverted, presumably as a response to subclinical infection. However, the antibody titres waned rapidly. This indicated that seroreaction expresses current or recent infection. Thus, the complement fixation test provides a reliable means of diagnosing pleuropneumonia of pigs and might be useful as a tool to control this disease.