In this letter to the editor the author takes exception to restrictions that an earlier researcher, Johnson, has suggested for the term teratogen. Johnson's efforts were to restrict the use of the term to substances that either adversely affect development at dose levels below those which are toxic to adults or to substances that are equally toxic to adults and embryos at maternally toxic levels. The current author feels these restrictions are scientifically constricting, partly because they base the labeling of a substance as a teratogen solely on its maternal toxicity. Maternal toxicity, itself a very difficult and complex issue, would then need redefining. A second difficulty with the proposed restrictions is that they somehow suggest that it is the maternal illness which produces the malformations in the offspring. Thirdly, the use of maternal toxicity as the basis on which to predict or limit potential developmental toxicity of either therapeutic or industrial agents may frequently be impractical. This author requests that other investigators would include descriptions of the signs of maternal as well as embryo/fetal toxicity for which observations were made and which were and were not affected. While the author agrees that misinterpretation of the term teratogen can be a problem, he doubts that redefining the central concept of teratology is the proper solution.