We report the development and application of an antibody-based nanoprobe for in situ measurements within a single cell. The nanoprobe has an antibody-based probe targeted to benzopyrene tetrol (BPT), a metabolite of the carcinogen benzo[a]pyrene (BaP) and the BaP-DNA adduct. Detection of BPT is of biomedical interest since this species can potentially serve as a biomarker for the monitoring of DNA damage due to BaP exposure and for possible pre-cancer diagnoses. The measurements were performed on the rat liver epithelial Clone 9 cell line, which was used as the model cell system. Nanoprobes were inserted into individual cells, incubated five minutes to allow antigen-antibody binding, and then removed for fluorescence detection. Prior to measurements, the cells had been treated with BPT. A concentration of 9.6 ± 0.2 × 10-11 M has been determined for BPT in the individual cells investigated. The results demonstrate the possibility of in situ measurements inside a single cell using an antibody-based nanoprobes.