It was shown that the polyreactive immunoglobulins of intact animal or human sera and the natural antibodies of these sera have different properties. Polyreactive immunoglobulins interact non-specifically with various antigens and this interaction is strongly dependent on an exposure of hydrophobic sites by antigens and, probably, by polyreactive immunoglobulins. Tween 20 and low temperature can substantially suppress this reaction. Various non-related soluble antigens can inhibit the binding of PRIG to any immobilized denatured antigen with similar efficiency. In contrast, natural antibodies interact specifically with appropriate antigens and this interaction can be suppressed only by the same or serologically similar competing antigens. Intact sera contain appreciable amount of polyreactive immunoglobulins, apparently much higher concentration than the concentration of natural antibodies. Biological functions of polyreactive immunoglobulins still remain unknown.