Immobilization of nine various proteins at cross-linked polyformylstyrene shell on 0.63-cm molded polystyrene beads intended for solid phase immunoassay has been investigated. The pH value of the protein solution and NaCl concentration substantially influence the density of the immobilized protein (DIP). The maximum values of DIP (up to 104 mu g/cm(2)) were obtained in pH = pI 0.1 M phosphate buffer and in 1 M NaCl, pH 3-4. The DIP decreased with the increase in the proteins molecular weight at a constant initial protein concentration. (C) 2000 Elsevier Science B.V. All rights reserved.