The motif for peptide binding to monoclonal antibody mAb A16, which is known to be directed against glycoprotein D of Herpes simplex virus type 1, was determined using two dedicated peptide libraries. As a starting point for this study we used an A-16 binding lead sequence, which had previously been obtained from a phage display peptide library (Schellekens et al., 1994). Binding studies with different length variants of this peptide identified a 12mer as a suitable lead compound for our library study. Two incomplete dedicated resin-bound synthetic peptide libraries were generated. Both consisted of 2 x 10(6) 12mers, in which positions were alternately fixed (amino acids identical to the lead sequence) and random. The libraries were screened with mAb A16 and beads with binding peptides were sequenced using Edman degradation. This resulted in a unique peptide binding motif, essentially comprising a 7mer core sequence. Comparison of the sequence of the natural epitope with the binding motif revealed that its sequence was identical to the motif except for one position. Substitution of a methionine in the natural epitope by a tyrosine or a phenylalanine at that position, as dictated by the motif, resulted in a peptide with an affinity for binding to mAb A16 about 50 times higher than that of the natural epitope. Thus, if a lead sequence is available, the use of incomplete, dedicated synthetic peptide libraries provides a fast and powerful tool for the detection of high affinity peptides.