Ribonuclease was treated in vitro with 73 chemicals, many of which are environmental pollutants, including inorganic, organic, and metal-organic chemicals, pesticides, and other biocides, alkyl and aryl industrial pollutants, and certain additional chemicals, to determine their effect upon enzyme activity. Palladium (II and IV) and gold (III) were the strongest inhibitors of RNase activity. Other strong inhibitors, in decreasing order of effect, were: sodium dodecyl sulfate, silver (I), EDTA, mercury (II), copper (II), thiram (fungicide), platinum (IV), malathion (pesticide), lead (II), and beryllium (II). Intermediate effects were found with other inorganic cations, many anions, and some other chemicals. A number of compounds of different chemical types caused no measurable effect. None of the chemicals tested caused a measurable activation of this enzyme.