AbstractA direct INT‐dehydrogenase assay (DIDHA) was developed to measure chemical toxicity as well as toxicant interactions. This test consists of measuring quickly and directly the dehydrogenase activity (i.e., INT reduction) with the aid of a double beam spectrophotometer, thus avoiding formazan extraction and centrifugation steps. The assay was used to determine the toxicity of heavy metals, phenol, α‐naphtol, and herbicides (Barban, propanil, swep, BIPC, and CIPC). It was also useful in demonstrating the protective effect of clays and humic acids with regard to herbicide toxicity as well as antagonistic and synergistic interactions among toxic