Industry continues to develop synthetic fibers for new technologies and as replacements for asbestos, a toxic and carcinogenic fiber. To determine whether the in vitro effects of the aromatic polyamide fiber, Aramid (Kevlar, Twaron), resembled those induced by asbestos, fibers were surveyed for (1) cytotoxicity as measured by total cell protein, and (2) proliferative capacity as measured by [3H]thymidine incorporation, colony forming efficiency (CFE), and ornithine decarboxylase (ODC) activity in two target cells of mineral dust-induced lung damage, hamster tracheal epithelial (HTE) cells and rat lung (RL90) fibroblasts. Results of cytotoxicity tests indicated that Aramid was as toxic to HTE and RL90 cells as were crocidolite and chrysotile asbestos when expressed on both an equal mass and equal fiber number basis. In HTE cells, Aramid caused a statistically significant increase in [3H]thymidine incorporation and CFE and produced a dose-dependent induction of ODC enzyme activity. Proliferative effects by asbestos or Aramid were not observed in RL90 fibroblasts. Thus, when tested over a respirable size range, Aramid exhibited many of the same effects on epithelial cells in vitro as did asbestos, including increased radiolabeled nucleotide incorporation into DNA and induction of ODC enzyme activity.