An investigation was conducted to determine if the micronucleus assay can distinguish between the carcinogen 4-chloro-o- phenylenediamine (5131602) (COP) and the two noncarcinogens 4-nitro- phenylenediamine (5307142) (NOP) and p-phenylenediamine- dihydrochloride (615281) (PPD). Male CD-1-mice were treated intraperitoneally with 400mg/kg COP, 500mg/kg NOP, and 100mg/kg PPD. Sampling was performed 24, 48, or 72 hours after injection. COP induced significant dose related increases in micronucleus frequencies at all three sampling intervals, with the peak response occurring at 24 hours. At that time the highest dose tested, 400mg/kg, induced 7.4 micronucleated polychromatic erythrocytes (MPCE) per 1000 polychromatic erythrocytes (PCE) compared to 1.9 MPCE of the control. Values for PCE/normochromatic erythrocyte (NCE) ratios in mice treated with COP were not significantly different from controls. Mice treated with NOP did not exhibit any dose related response in micronuclei induction over the three dose levels tested when sampled at 24, 48, or 72 hours. PPD did not cause a significant dose or sampling time related response in micronuclei induction. Ratios of PCE/NCE in bone marrow cells of mice exposed to PPD exhibited significant decreases at the highest dose, 100mg/kg, at sampling times of 24 and 72 hours.