SUMMARYComparisons of antigenic preparations and antisera of isolates of L.persooniiby agar gel diffusion tests indicated a relatedness between isolates pathogenic on President plum that was not shared by the nonpathogenic isolates. However, 2 sister monoascospore isolates, one pathogenic and the other nonpathogenic, were most closely related based on the presence of at least 4 common precipitin bands. Differing degrees of relationship among the isolates were evident from common precipitin bands in cross reactions and the absence or weakness of these bands in reciprocal absorption tests.The identity of an isolate ofR. quercinawas confirmed by its serological comparison with a known isolate of this species.The apparent absence of common antigens between isolates ofL. persooniiandR. quercinafurther justifies their separation into different genera.