The invention concerns a method for the determination of a partner in an immunological reaction based on an immunoassay in which one of the reaction partners is present in a solid phase, wherein a polyethylene oxidepolypropylene oxide block copolymer having the formula I ##STR1## is used as the surface-active agent in which a can have a value between 40 and 150 and b a value between 10 and 50, which is characterized in that a hydrophilic block copolymer composition having the formula I is used which in a 1 % aqueous solution (weight/volume) has a surface tension of at least 45 mN/m and in which the average molar ratio of ethylene oxide groups to propylene oxide groups (2a/b) in the composition is at least 5.8. In addition the invention comprises a reagent for carrying out the above method.