Platelet factor 4 (PF4) is a hydrophobic, alpha-granule protein with potent antiheparin activity. It also binds to a chondroitin sulfate- containing proteoglycan (PG) isolated from platelets. In order to evaluate further the relationship between PF4 and the chondroitin sulfate-containing proteoglycan in resting platelets, the PF4-binding proteoglycan from human platelets has been purified using purified PF4 as an affinity ligand and used to prepare polyclonal antiserum. Two antisera have been characterized: one reacts primarily with chondroitin sulfate (CS), the other reacts with the protein core of the platelet proteoglycan after chondroitinase AC digestion. PF4 and PG core protein antigen are present in separate, dissimilar precipitin arcs when triton- solubilized platelets are analyzed by crossed immunoelectrophoresis using polyclonal antisera to purified PF4 and PG. PF4 was demonstrated in a complex with a separate chondroitin sulfate antigen by crossed immunoelectrophoresis (CIE) experiments in which either anti-PF4 or anti-CS antisera was incorporated in the intermediate gel. Both the PF4- chondroitin sulfate complex and the proteoglycan are secreted from platelets when fresh, washed human platelets are stimulated by human alpha-thrombin. This second antigen may represent the PG after posttranslational modification of a precursor form of the proteoglycan.