Composite materials comprised of the electrically conducting polymer, polypyrrole, with a variety of biologically active molecules, e.g. proteins or polysaccharides, are emerging as a novel class of "smart" biomaterials. In the present work we have studied the utility of a heparin-polypyrrole composite as a substrate for human umbilical vein endothelial cell (HUVEC) growth. We found that the polymer composites were well suited to support cell attachment and growth; displaying low surface hydrophobicity (water contact angle of approximately 20 degrees ) and roughness, Rq of approximately 10-12 nm. Doubling times for HUVEC on heparin-polypyrrole were greater than observed for gelatin-coated tissue culture polystyrene (44 and 36 h, respectively), however, the cells did proliferate to cover the polymer in an even monolayer. The initial mechanism of attachment and subsequent proliferation of HUVEC on heparin-polypyrrole was critically dependent on the presence of the serum adhesion glycoprotein vitronectin. Polymers that were composed of polypyrrole and sodium nitrate were more hydrophobic than heparin-polypyrrole and they did not support HUVEC growth. Given the relative ease with which these polymer composites can be electrochemically synthesized, the diverse range of cellular "signal agents", e.g. growth factors, that can be incorporated within them, and the high degree of control that can be achieved in the release-surface exposure of these agents, we suggest that polypyrrole composites could serve a useful role as "smart" biomaterials in the near future.