Cells were plated and grown to about 80% confluence on cover slips. The cover slips were treated with 3% hydrogen peroxide plus 0.1% NaN 3 to block endogenous peroxidase activity, and then incubated with phages. After the cover slips had been washed and fixed with 3% paraformaldehyde, they were incubated with HRPlabeled mouse anti-M13 mAb and treated with peroxidase substrate. For the peptide binding assays, 30 mg/ml FITC-labeled SP5-2 or control peptide was added on each cover slip and incubated. They were counterstained with Hoechst 33258 (Molecular Probes, OR, USA) and mounted with a mounting solution (Vector, CA, USA). The cells were then examined under a Leica universal microscope. The images were merged using the SimplePCI software (C-IMAGING, PA, USA).