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Mycelial and culture filtrate antigens from 10 progressive and 10 fluctuating isolates of Verticillium albo-atrum were analysed by crossed immunoelectrophoresis and rocket-line immunoelectrophoresis using antisera raised against pooled mycelial and pooled culture filtrate antigens from 5 isolates of each type. In crossed immunoelectrophoresis, pooled antigens produced precipitin lines corresponding to about 30 mycelial and 25 culture filtrate antigens with homologous antisera. Cross-absorption of antisera with heterologous antigens indicated that the culture filtrate preparations from the two types of isolate were identical and that the mycelial extracts from the two groups differed in one antigen, antigen ‘21’. This antigen was present in mycelial preparations from 9 out of 10 progressive isolates but was either absent or present in low concentration in 7 out of 10 fluctuating isolates. Antigen ‘21’ was also present in culture filtrate preparations from 15 of the 20 isolates.
On the basis of the precipitin patterns of antigen ‘21’, three serological groups of V. albo-atrum may be described. It is suggested that the virulence of progressive isolates might be associated with the production of high concentrations of this antigen.