Several concentrations of the insecticide dieldrin were tested on in vivo bone marrow cells of STS mice and in vitro human embryonic lung cells of the WI-38 cell line. The insecticide caused pronounced mitotic inhibition and produced a 2 to 3-fold increase in chromosome abnormalities such as breaks and fragments in concentrations as low as 1 mg/kg in STS mice bone marrow cells, and 1 mug/ml in human WI-38 cells. The chemical also produced chromosome interchanges and rings. A few cells from the control groups exhibited breaks and fragments but not exchange figures and rings. Cytotoxic studies using the WI-38 cell line revealed dose-response and time-response reactions to dieldrin. Cell toxicity was most pronounced at the 50 mug/ml treatment level, producing 100 percent cell degeneration within 6 hours.