2,4-Dichloro-6-aminophenol (DCAP) is a major metabolite of the herbicide 2,4dichloro-6-nitrophenol ammonium (DCNPA). In our previous studies this metabolite, but not the parent compound, was found to induce sister chromatid exchanges (SCEs) and micronucleus (MN) formation in Chinese hamster lung fibroblasts (V79 cells). Further studies were carried out to determine whether DCAP can also induce structural chromosomal aberrations (CAs) in cultured V79 cells. The assay was performed under three different treatment conditions: 1) 3 h treatment followed by 17 h post-treatment incubation, 2) 12 h treatment, and 3) 20 h treatment with multiple sampling times. Results showed that CAs were induced by DCAP in V79 cells. The aberrations were mainly chromatid types (gaps, breaks, and deletions), triradials, quadriradials, and complex rearrangements. However, induction of CAs appears to be dependent on the treatment condition. The 3 h treatment following a 17 h posttreatment incubation gave rise to 24% aberrant cells (excluding gaps) and a total of 44 aberrations per 100 metaphases, while the 12 h treatment only gave a borderline response and the 20 h treatment did not yield a significant increase of CAs at any concentration tested. It seems that a short-term treatment with higher concentrations is a better treatment protocol for genotoxicity studies of certain cytotoxic chemicals such as DCAP. Results of this study further indicate that the herbicide DCNPA is potentially hazardous to the exposed population.