Heparan sulfate chains of proteoglycans bind to and regulate the function of a wide variety of ligands. In myoblasts, heparan sulfate proteoglycans modulate basic fibroblast growth factor activity and regulate skeletal muscle differentiation. The aim of this study was to identify endogenous extracellular ligands for muscle cell heparan sulfate proteoglycans. [(35)S]heparin ligand blot assays identified a 33/30 kDa doublet (p33/30) in detergent/high ionic strength extracts and heparin soluble fractions obtained from intact C(2)C(12) myoblasts. p33/30 is localized on the plasma membrane or in the extracellular matrix where its level increases during muscle differentiation. Heparin-agarose-purified p33/30 was identified as histone H1. In vitro binding assays showed that histone H1 binds specifically to perlecan. Immunofluorescence microscopy showed that an extracellular pool of histone H1 colocalizes with perlecan in the extracellular matrix of myotube cultures and in regenerating skeletal muscle. Furthermore, histone H1 incorporated into the extracellular matrix strongly stimulated myoblast proliferation via a heparan-sulfate-dependent mechanism. These results indicate that histone H1 is present in the extracellular matrix of skeletal muscle cells, where it interacts specifically with perlecan and exerts a strong proliferative effect on myoblasts, suggesting a role for histone H1 during skeletal muscle regeneration.