The effect of pH of the blocking solution on the non-specific sorption of peroxidase conjugates to the 0.63-cm formylated polystyrene beads was studied. Anti-G-HRP and BSA-HRP sorption was shown to dramatically depend on pH of blocking protein solutions and was minimum at pH=pI. Sorption of G-HRP weakly depended on pH of the blocking buffer. the blocking efficiency of proteins on unmodified beads, as well as of Tween-20 on formylated beads appeared to be pH independent. the optimized blocking step resulted in a multiple decrease in the non-specific sorption of conjugates on formylated beads.