When fresh platelets were incubated in serum to which heat-aggregated IgG was added, the increase of platelet-associated IgG was paralleled by increased levels of platelet-associated IgM, IgA and albumin as determined by ELISA. The increase was observed regardless of whether the platelets were quantified by conventional platelet counting or with the help of 51Cr-labelling, which was done to avoid potential counting errors induced by platelet fragmentation. In thrombocytopenic patients a correlation between in vivo bound platelet-associated albumin and IgG could be confirmed. It was concluded that aggregated IgG induces an increased binding of serum immunoglobulins and albumin to platelets in vitro.