SRI used the L5178Y mouse lymphoma cell forward mutation assay to determine the mutagenic activity of 63 coded chemicals from 16 chemical classes. Replicate experiments were performed to assess the reproducibility of the assay within the laboratory. The evaluations (positive or negative) of the first two repeat experiments with the chemicals were the same for 116 (87%) of 134 tests. Evaluational differences between the first two experiments were fewer in the presence of induced S9 (6 tests) than in the absence of S9 (12 tests). The most commonly observed variability was the magnitude of positive mutagenic responses; this may be attributed to factors such as compound solubilities, S9 activation conditions, and differential recovery of mutant cells. Some consistency was observed in the responses of compounds of various chemical classes. Generally, antibiotics (ABO) and the azo dyes, azoxy and hydrazo compounds, diazoalkanes, nitriles and azides (AZO), were mutagenic with S9; alkyl, acyl, and aryl halides, halogenated ethers, and halohydrins (HAL) were more strongly mutagenic with than without S9; and monofunctional polycyclic aromatic hydrocarbons and fluorenones (PAH) were mutagenic only with S9. Amine-1-oxides (AMO), alkyl and aryl epoxides (EPO), and nitroalkanes, nitroaromatics, nitroquinolines, nitrofurans, and nitroimidazoles (NIT) were mutagenic with and without S9; amides, sulfonamides, aromatic amines, aliphatic amines, hydroxylamines, and benzidine and its derivatives (AMI) were mutagenic without S9; and methyl carbamate (the only monofunctional carbamate) and thioureas (CBM) induced a negative response under both conditions.