The phytotoxins seiridin (SEI) and iso-seiridin (ISE), two delta alpha,beta-butenolides produced in vitro by Seiridium cardinale, S. cupressi, and S. unicorne, as well as their derivatives obtained by chemical modification of each toxin, were analyzed for their bioactivity. The effects each compound on host and non-host plants and their antimicrobial activity on bacteria were investigated. The toxicity of both seiridins (SEIs) decreased in the derivatives with modifications of the gamma-lactone ring or with acetylation of the hydroxy group of the aliphatic side chain at C-4. Shoot tissues of Cupressus macrocarpa artificially infected by S. cardinale leached electrolytes more than those of C. sempervirens and C. arizonica. Electrolyte loss from shoot tissues of cypress plants treated with each derivative also decreased for most of them. Seed germination was not affected by SEI and ISE derivatives. Inhibition of root growth of three herbaceous test plants was studied. SEI and 3,4-dihydro SEI were active to germlings of lettuce. No ISE derivative affected root growth of lettuce and oat germlings. Reduction was observed on roots of radish germlings treated with acetyl ISE or 3,4-dihydro ISE. No derivative of SEI or ISE elicited hormone-like activity as SEI did. Antibacterial activity shown by SEI and ISE at 150 microM accounted for both hydrogenated derivatives of SEI. The integrity of the delta alpha,beta-unsaturated-gamma-lactone ring and the location of the hydroxy group in the heptyl side chain are features of importance in biological activity of the two butenolides.