An agar-plaque technique for titration of vibriocidal antibodies was studied. The elaborated technique is based upon application of microdrops of serum diluted with complement on the surface of a nutrient agar plate with incorporated live vibrios. After incubation at 37dGC for some 4 hours clear plaque:; in the layer of confluent bacterial growth at the sites for the serum-complement mixture indicate the complement-dependent vibriocidal effect of serum antibodies. This spot method is more sensitive than bacterial agglutination and conventional vibriocidal tests in fluid medium and it shows the same reproducibility as these methods. A modification of the spot method was developed employing radial pre-diffusion of the antibodies before addition of complement and the subsequent multiplication of bacteria. The antibody concentration of the tested sample was found to be correlated to the area of the growth inhibition zone. This correlation is utilized for the establishment of a zone vs concentration curve for a standard serum which can be used for the evaluation of test samples. By means of the radial diffusion method 20 per cent or less vibriocidal antibody differences could be discriminated and the sensitivity was as high as that of the spot agar-plaque method. With the spot as well as the diffusion techniques vibriocidal activity could be demonstrated for 7S as well as 19S antibodies and in absorption experiments serotype specific vibriocidal antibodies were revealed in addition to cross-reacting antibodies.