AbstractA general protocol, modified from the one described by Clive and Spector (Mutat Res 31:17–29, 1975), was followed by two laboratories, Litton Bionetics, Inc., and SRI International, to evaluate 63 coded chemicals from 16 chemical classes for mutagenic activity at the thymidine kinase locus in L5178Y TK±3.7.2C mouse lymphoma cells. The general protocol is discussed. Some procedural variations introduced by both laboratories are described and discussed in terms of their potential effect on the comparative results of the assay. Also included are the chemical structures, molecular weights, and functional classifications of the 63 chemicals. The assay appeared to tolerate the specific procedural variations in each laboratory without changing its reliabili