Cassia obtusifolia and its seeds, common contaminants of agricultural commodities, are toxic to cattle and poultry. Toxicity has been attributed to anthraquinones which are major constituents of C. obtusifolia, but studies of the subchronic and chronic toxicity of naturally occurring anthraquinones are limited. To investigate the subchronic (greater than 30 days) toxicity of C. obtusifolia seed, ten rats/sex were fed diets containing 0, 0.15, 0.50, 1.5 or 5.0% C. obtusifolia seed for 13 weeks. Intermittent mild diarrhea was found in high-dose animals and body weights of high-dose males were decreased to week 10. Myeloid hyperplasia with peripheral leukocytosis, thrombocytosis and mild anemia were found in males and females fed diets containing greater than or equal to 0.50% C. obtusifolia seed. Leukocytosis resulted from neutrophilia, whereas peripheral lymphocyte counts were unaffected. Lymphoid hyperplasia and/or histiocytosis were found in the mesenteric lymph nodes in groups fed C. obtusifolia seed. Thus, a dietary 'no observable effect level' for subchronic ingestion of C. obtusifolia seed in rats was less than 0.15%.