This supplement to Survey of Compounds Which Have Been Tested for Carcinogenic Activity has retained the format of the previous two editions with some minor modifications. Radioactive materials have not been included on this occasion because it was felt inappropriate to classify these materials on the basis of their chemical structure rather than physical properties. It can be seen that the proportion of carcinogens among the new compounds (21%) is similar to that found before (23%). In view of the greatly increased interest in carcinogenesis as a problem of human environmental cancer, it is of great importance to analyze the significance of this proportion.