Factors affecting the sensitivity of the passive haemagglutination method for serotyping campylobacters have been studied. The concentration of red blood cells during the haemagglutination stage of the procedure markedly affected the titer obtained. An increase in concentration of red blood cells resulted in a lower titer, with titers being inversely proportional to red blood cell concentration. No differences in titer were observed when erythrocytes were sensitized at a range of pH values between pH 5.0 and pH 8.0. The time required for antigen extraction and for red blood cell sensitization was shown to be 15 min each, thus resulting in a reduction in the time required for serotyping. Furthermore, use of avian erythrocytes enabled the haemagglutination reactions to be read after incubation for only 1 h. Combining these procedures with a rapid slide haemagglutination test enables a single worker to serotype over 100 C. jejuni and C. coli isolates within 1 working day.