The agricultural antibiotic monensin caused herbicidal injury to 1-to 2-week-old seedlings of seven weed and two crop species when applied at M as a foliar spray in the greenhouse. The non-ionic surfactant tergitol TMN (0.1 ml litre-') aided foliar absorption of monensin and increased herbicidal injury. Fresh weight reductions ranged from 20 to 75% (compared to control seedings treated with tergitol alone) 65 h after spray application of M monensin with tergitol. When root-fed at lo-" M, monensin (without tergitol) caused death in all nine species within 24-72 h after treatment. Monensin supplied to roots at lo-' M caused fresh weight reductions in spurred anoda, velvetleaf and prickly sida of 43, 32 and 23%, respectively. Substantial growth effects also occurred in hemp sesbania and sicklepod (33 and 15% reductions in fresh weight, respectively); jimsonweed and johnsongrass had the least fresh weight reductions. Injury included chlorosis, necrosis, desiccation and leaf abscission. Cotton and okra were the most tolerant of all species tested; however, three malvaceous weeds were severely damaged by the compound. When root-fed at M, monensin caused limited injury. Monensin exhibited some degree of selectivity among these crop and weed species and caused greater injury in light-treated than darktreated plant tissues.