One normal human lymphoid cell line was treated continuously with the fungicide Folpet (N-trichloromethylthio-phthalimide) at 0.5, 1, 2, and 4 microgram/ml of media and one Burkitt cell line was pulse-treated with the same compound for 15 min at 2.5, 5, 10, and 15 microgram/ml. Various times after treatment, cell viability counts and chromosomes analyses were made. Severe cell growth inhibition and an increase in chromosome aberrations were observed in both cell lines. There were two types of induced chromosome aberrations. Aberrations type A appeared in cultures treated with lower doses and were characterized by chromosome or chromatid gaps and breaks and also infrequently seen exchanges and dicentrics. In the later stages of the experiment, the rate of growth and aberrations returned to control levels. Chromosome aberrations type B appeared in early samples of cultures treated with higher doses. The whole complement of chromosomes was affected. The aberrations included chromosome stickiness, despiralization and pulverization. The cultures died off within two days.