The adsorption at silica and methylated silica of genetically engineered derivatives of Z and its dimer ZZ, where Z is a hydrophilic synthetic IgG binding domain derived from staphylococcal protein A, was studied with in situ ellipsometry. The protein modifications consisted of introducing short peptide stretches near the C-terminus of the protein, Using this approach, oligopeptide stretches containing hydrophobic tryptophan (Trp) or isoleucine (Ile) [(AlaTrpTrpPro)(n) or (AlallelllePro)(n) (0 less than or equal to n less than or equal to 2), denoted T-n and I-n, respectively] were introduced in the protein. For comparison, the adsorption of the inserted peptide stretches (T-n and I-n), as well as of Trp and lie oligomers, was investigated. Increasing the number of Trp residues resulted in increased adsorption for ZZT(n), ZT(n), T-n, and Trp(n). At a given number of Trp residues, the adsorbed amount of the ZZ derivatives is larger than that of the peptides but about the same as that of the Z proteins, Analogous although somewhat smaller effects were obtained for the Ile-derivatized proteins. These results are discussed in terms of the "block copolymer" nature of the proteins. Theoretical calculations using a mean-field lattice model for block copolymer adsorption gave a qualitative agreement with the experimentally obtained results.