A phase-resolved fiberoptic antibody-based biosensor is desci ibed, which combines the biochemical specificity of antigen-antibody reaction, the high sensitivity of laser excitation, the versatility of fiberoptic sensors, and the selectivity of phase-resolved detection. The device can differentiate benzo(a)pyrene and benzo(a)pyrene tetrol, a DNA-adduct product of the carcinogen benzo(a)pyrene. The limit of detection of benzo(a)pyrene tetrol is in the attomole range (10-18 moles).