Plant weight and contents of chlorophyll, ionic thiocyanate (SCN(-)), and hydrogen cyanide (HCN) were determined in cabbage (Brassica oleracea var.capitula L. cv. Early Greenball), bean (Phaseolus vulgaris L. cv. Contender), and tobacco (Nicotiana tabacum L. cv. Delhi 76) grown hydroponically in modified Hoagland's nutrient solution with six concentrations of SCN(-) (supplied as KSCN) (0, 5, 25, 50, 100, and 200 mg/liter). Whereas tobacco plants did not grow with any level of SCN(-) in the culture solution, beans grew with 5 mg/liter and cabbages grew with between 5 and 50 mg/ liter. Increasing levels of SCN(-) in the culture solution resulted in decreased growth and chlorophyll content, accompanied by consistently increasing amounts of SCN(-) in cabbage. Small amounts of HCN found only in tissues of cabbage were not influenced by levels of SCN(-). The greater insensitivity of cabbages to the presence of SCN(-) compared with beans is apparently related to the presence of endogenous glucosinolates which are capable of being degraded into SCN(-). Accumulation of SCN(-) and occurrence of leaf chlorosis in cabbage and beans and death of tobacco plants supplied with SCN(-) in hydroponic culture confirm the capacity of SCN(-) as an allelopathic agent, but its effect mechanism in ecology needs to be demonstrated.