Eighteen linear antigenic sites were found in cytochrome P450 101 (P450cam) from Pseudomonas putida by the peptide scanning method. These sites accounted for about 30% of the protein sequence. We found no sequences that completely coincided with the antigenic sites of P450cam in cytochromes P450 from other sources. The linear B-epitopes of P540cam were mainly localized on the boundaries separating the elements of the secondary structure. Seventeen of eighteen antigenic sites were found to be on the protein surface and accessible to water molecules. Many functionally important sites or amino acid residues of the P450cam molecules coincided or were in close proximity to the linear B-epitopes found.