The "classical" Merrifield method was used to synthesize over 500 peptides using Boc-benzyl strategy. The peptides were prepared either manually or on a Beckman 990B synthesizer or an Applied Biosystems 430A synthesizer. Each coupling of Boc amino acid to the growing peptide on the resin was monitored with the ninhydrin reaction. Couplings were considered "incomplete" if there was 99% or less coupling and "high incomplete" if there was 98% or less coupling. The efficiency of coupling was evaluated in regard to the specific amino acids involved in the coupling reaction and to the length of the peptide at the time of the coupling. The most difficult carboxyl-reacting amino acids were histidine, threonine, arginine, valine, isoleucine and glutamine; the most difficult amine reacting residues were glutamine, leucine, alanine, arginine and isoleucine. The number of "incomplete" and "high incomplete" couplings and the total number of monitored couplings of each of the 20 carboxyl-reacting amino acids when reacting with each of the 20 amine-reacting residues were tabulated. Coupling efficiencies decreased with the length of the peptide. The conclusion of this study is that, with the chemistries and methods used in this group of peptides, no amino acid coupling can be predicted to be complete with a single coupling reaction. The study points to the need for on-line determination of coupling efficiency during the synthesis in which a recoupling step is initiated when the first coupling is incomplete.