In vitro exposure of adult rat alveolar macrophages to either paraquat or diquat resulted in concentration dependent cytotoxicity (cell death). The herbicide paraquat, however was statistically significantly more potent toward these cells than was diquat. The LC50 value for paraquat (8‐h exposure, 37°C) was determined to be 0.94 mM [95% confidence interval (C.I.) 0.79–1.12 mM], whereas the corresponding LC50 value for diquat was 1.97 mM (C.I. 1.58–2.51 mM).