Abstract <p>The criteria for searching for proteins that can substitute for the osmotic function of serum albumin (SA) in the blood of albumin-free fish that have lost albumins during the evolutionary process have been developed: (1) high negative charge of the proteins substitutes (<i>q</i>^<b>–</b>) as a measure of their high osmotic activity and/or (2) the ability to remodel with the formation of particles with high <i>q</i>^<b>–</b>. In albumin-free fish, these criteria are met by multiple serum α₁-globulins: α₁-antitrypsin (A1AT); heat acclimation protein Wap65 or hemopexin Hx; and apolipoproteins A within high-density lipoproteins (HDLs). Their status as factors with high osmotic activity comparable to mammalian albumins is confirmed by experimental data from various authors. A comparison of the organization of serum α₁-globulins in albumin-free fish and mammals with analbuminemia allows us to identify strategies in fish (1) to enhance the negative charge of A1AT and Wap65, which is not typical for A1AT and Hx in humans, and (2) to the dominant content of HDL lipoproteins in the blood of fish, which, during remodeling in the experiment and in vivo, form particles with high <i>q</i>^– and high water-binding activity. The results confirm the concept of the stability of the protein system of plasma in Vertebrata, capable of compensating for the functions of individual proteins lost during the evolutionary process and adapting to environmental conditions due to adaptive changes in the organization of proteins, including by changing their surface charge, which affects osmotic activity.</p>