Although the mineral talc is a specific chemical substance, H2Mg3(SiO3)4, commercial talc is a material which is relatively constant physically but not chemically. The authors show analyses of 51 different commercial talcs. Appropriate amts. of 7 of these were injected into the peritoneal cavities of guinea pigs and the reactions in the tissues observed by sacrificing the animals at varying periods up to 15 mos. Character-istically, the material was taken up by macrophages and giant cells in nodules which had a minimal amt. of fibrosis. The maximum amt. of fibrosis was observed in those samples of commercial talc which had the least amts. of carbonate in the compound. Suggestion is therefore made that in using commercial talcs effort should be made to avoid the use of low carbonate-containing commercial talcs because of the possibility of producing excessive amts. of pulmonary fibrosis.