The nature of the binding of specific saccharides to Abrus precatorius agglutinin (APA) was studied by ultraviolet difference spectroscopy. Upon binding of saccharides, APA displayed difference spectra with maxima at 291-292 nm and 284-285 nm. Such spectra suggest that the state of the tryptophan residue closely associated with the saccharide-binding activity of APA is perturbed by the binding of a saccharide. The difference spectra value (delta epsilon) increased with increasing saccharide concentration. From the increase in delta epsilon at 291-292 nm, the association constant (Ka) was obtained for the binding of individual saccharides to APA. Lactose bound to APA with the highest affinity among the saccharides examined and its Ka value (8.3 X 10(3) M-1 at pH 7.0 and 25 degrees C) was approximately four times as large as that of galactose (2.2 X 10(3) M-1). Raffinose and methyl beta-galactopyranoside showed larger association constants than galactose. Galactosamine, N-acetylgalactosamine and 2-deoxy galactose were found to bind with APA with fairly low affinity. The shape of the lactose-induced difference spectrum changed with pH and the spectrum in the acidic region showed characteristic broadening of the difference maximum peaks. The affinity of lactose to APA was nearly equal in the range of pH 6-8, but decreased outside this pH region and with increasing temperature.