The accurate identification of Fusarium species can take 2-3 weeks. Preliminary exoantigen studies indicate that a mature culture suspected of being a Fusarium species may be immunologically identified 48 h after receipt. Exoantigen extracts of 10-day-old slant cultures of Fusarium chlamydosporum, Fusarium moniliforme (= Fusarium verticilloides), Fusarium oxysporum, Fusarium proliferatum and Fusarium solani and partially purified reference homologous and heterologous shake culture extracts (6-week-old) were reacted against rabbit anti-F. chlamydosporum, F. moniliforme, F. oxysporum, F. proliferatum and F. solani sera, in a micro-immunodiffusion procedure. The results indicated that all the strains belonging to a given species produced 1-3 bands of identity only when tested against its homologous antiserum and reference antigen. No cross-reactions were observed with the heterologous antisera. Furthermore, extracts from isolates of Fusarium dimerum, Fusarium equiseti, Fusarium roseum complex, Acremonium species, Cylindrocarpon, Fonsecaea pedrosoi and Trichoderma species did not react with any of the prepared Fusarium species' antisera. Our data suggest that the exoantigen procedure is a rapid and reliable tool for the accurate immuno-identification of the medically important Fusarium species studied.