Submicrometer fiber-optic biosensors have been developed and used to measure toxic chemicals within single cells. Optical fibers that have been pulled to a distal-end diameter of less than 1 micrometer are coated with antibodies to selectively bind the species of interest. This paper describes the use of these fibers to selectively measure the concentration of benzo[a]pyrene tetrol (BPT), a metabolite of benzo[a]pyrene, within individual cells of two different cell lines, human mammary carcinoma cells and rat liver epithelial cells. The results from these measurements have been used to determine the sensitivity, reproducibility, and usefulness of these nanosensors. The detection limit of these biosensors has been determined to be 0.64 +/- 0.17 x 10(-11) M for BPT.