The Penner serotyping system, based on detection of heat-stable antigens with a passive haemagglutination technique, was used in studies on Campylobacter epidemiology in poultry. Preparation of specific antisera by absorption allowed the use of pooled antisera. Over 80% of the Campylobacter isolates were typable with this modified Penner serotyping system. Typability of strains was clearly affected by storage of the strains before actual typing. Extracted antigens appeared to be stable for at least 6 months at 4 degrees C. Therefore, it is advisable to store extracted antigens from freshly isolated Campylobacter strains instead of reculturing frozen-stored strains, when actual typing cannot be performed directly after primary isolation. Untypability of isolates may partly be explained by the detection of Campylobacter serovars not yet represented in the serotyping system. Experiments on repeated serotyping of several Campylobacter strains did not suggest any serovar instability within the strains.