We have used a tris(4-methoxyphenyl)-phosphonium compound as a hapten to elicit catalytic antibodies that selectively remove trityl protecting groups at neutral pH. One antibody, 37C4, was characterized kinetically with a number of trityl substrates. The rate enhancement was consistently near 200; the Km was approximately 30 microM for the methoxytrityl substrates. Compounds with no methoxy substituents on the trityl group were not hydrolysed by the antibody. No decrease in the rate of reaction was detected through 21 turnovers, which suggests that the presumptive trityl cation formed during the cleavage reaction does not alkylate the antibody binding pocket. The rates of the background and antibody-catalysed reactions both increase logarithmically with decreasing pH, implying that general acid catalysis is not involved: further studies will test this assumption. The favoured mechanism for the catalytic activity of antibody 37C4 is charge complementarity in the binding site stabilizing a positively charged intermediate(s) in the cleavage reaction. The coding sequence for 37C4 is being cloned into a phage lambda vector in preparation for site-directed mutagenesis to improve the catalytic efficiency of the antibody.