An antiserum was made against Plasmodiophora brassicae, the causal agent of club root of cabbage. A semipurified suspension of spores of P. brassicae was used as antigen, obtained by filtration and Percoll gradient centrifugation of infected roots. Crude root suspension samples were bound to a nitrocellulose membrane and tested by a dot immunobinding assay. The individual steps of the serological procedure were examined with a scanning electron microscope. The surface of the resting spores of P. brassicae, race 7, appeared smooth, while the dot immunobinding processed spores had a heavy, irregular coating, which was demonstrated to originate from incubation in the primary antiserum. Normal serum did not give rise to a coating on the resting spores. The antiserum of P. brassicae did not react with surface antigens of resting spores of Polymyxa. Further, no cross reaction with other common root pathogens such as Pythium ultimum, Rhizoctonia solani, and Fusarium oxysporum was observed. With antiserum prepared against sproe surface antigenic determinants the dot immunobinding technique can be used as a routine test for detection of infection of P. brassicae in host plants and in bait plants (used as indicators of soil infestation). The sensitivity obtained was within the range permissible for a routine test.