Further investigation of the monoclonal antibody 16G3 has revealed that it not only couples activated amino acids to form dipeptides with high turnover rates but also couples an activated amino acid with a dipeptide to form a tripeptide, as well as an activated dipeptide with another dipeptide to give a tetrapeptide. The catalytic rates for these reactions greatly exceed the background rate of ester hydrolysis providing average yields of 80% within the assay time of 20 min. Importantly, the amount of product inhibition is low, allowing for high yields of products using multiple addition of substrates to the same antibody reaction mixture. A sequential mechanism is employed by 16G3 for dipeptide coupling, and this mechanism appears to hold for the formation of the other peptides. High catalytic selectivity is observed for the nucleophilic alpha-amino group of an alpha,beta-diamino nucleophile and for the para substituent on the activated ester, traits that are consistent with hapten design. The former chemoselectivity is crucial for the condensation of fragments which are unprotected at the epsilon-amino group of lysine.