Summary The mechanism of action of the phenylsulphonyl carbamate herbicide (asulam) was examined in celery tissue cultures to see whether the mechanism involved an inhibition of folic acid synthesis. When asulam was included in the nutrient medium, growth of the celery cultures was reduced but not entirely inhibited even at a concentration of 250 μm. Growth was also reduced if a phenylcarbamate herbicide (barban), or sulphanilamide were included in the medium. The addition of folic acid, or 4‐amino benzoic acid, which is a precursor of folic acid, almost totally reversed the inhibitory effect of the asulam and sulphanilamide, but not of barban, whereas the addition of thymidine, methionine, serine or adenine, all of which are formed from folic acid, did not reverse the inhibitory effect of asulam, sulphanilamide or barban. Nevertheless the removal of the growth inhibition of asulam by folic acid or 4ABA suggested the mechanism of action of asulam was similar to that of a sulphanilamide. This mechanism of action in celery cell cultures appears to be similar to that found for asulam in intact plants of a range of species.