The relationship between the toxicities of crude extracts and purified toxins of Fusarium spp. belonging to the section Sporotrichiella has been assessed. Toxicity was determined on the basis of death of Artemia salina larvae and of viability and blastogenic response of bovine and human lymphocytes. Trichothecene-producing strains of Fusarium sporotrichioides and Fusarium poae were toxic to A. salina and to lymphocyte blastogenesis. A strain of Fusarium tricinctum, producing visoltricin and chlamydosporol, induced differentiated activity in different bioassays (toxicity to A. salina but only minor activity against lymphocyte blastogenesis). Other, non-toxin-producing strains of Fusarium chlamydosporum, F. poae, and F. tricinctum were not active in the tested biosystems.