Two laboratories tested multiple forms of xylene for their developmental toxicity hazard potential (A/D ratio) by means of the hydra assay. The three isomers, as well as a solution of mixed xylenes, all interfered with development (D) at or near to concentrations that also were toxic to adult (A) hydra. The A/D ratios ranged from 1 to 2 in hydra as they had in conventional tests made in pregnant laboratory animals. Each testing laboratory concluded that xylenes were not primary developmental hazards but coaffective agents capable of disrupting development only at or near to concentrations also toxic to adults. In each instance every xylene tested interfered with the same stage or developmental sequence and in a concentration-related manner. The hydra assay may be useful for establishing priorities to test agents in a more elaborate system, but substances less soluble than xylene may exceed the test's applicability.