The interactions at pH 5.5 of the water-soluble poly(N-methacryloyl-4-aminosalicylic acid) (poly(4-HMA)) with small amounts of Ca2+ and Zn2+, taking into consideration the statistical stereochemical distribution of the monomeric methacrylic units, are analysed by an ultrafiltration technique. The values of the interaction constants for the complexation of the salicylic side groups with each metal ion at these experimental conditions are given as a function of the ionic strength, and the results obtained indicate that Zn2+ interacts with the polymer 16 times better than Ca2+.