Fascin is a 55 kDa actin filament bundling protein first isolated from sea urchin egg actin gels which consist of actin-fascin bundles cross linked by spectrin. Purified actin and fascin produce bundles when mixed in vitro, with an actin-fascin stoichiometry of 4.6 to 1 at saturation. Cloning of sea urchin fascin cDNA revealed homology to an uncharacterized Drosophila protein called singed. The singed phenotype, misshapen bristles and female sterility, can be explained by a lack of actin bundles and it has been shown recently that the singed gene product actually bundles actin. Mammalian fascins have now been cloned from humans and mice. Murine fascin is highly expressed in brain; immunofluorescence reveals fascin localized to filopodia extending from the leading edge of hippocampal neuron growth cones. Our studies of actin-fascin with electron cryomicroscopy and computer reconstruction reveals a number of interesting observations: firstly the recombinant fascin protein forms large and crystalline aggregate with actin in a way different from that of the native bundle. This can be caused by the presence of thioredoxin in the recombinant protein; the recombinant protein has a slightly different structural conformation as the native one or we have not yet reached the appropriate chemical conditions to form bundles. Secondly, the similarity and difference in the projection structures between the native and recombinant fascin and actin complex provides an interesting system to study both of them at a higher resolution in three dimensions in order to understand how the bundle is formed. Thirdly, since the major protein mass of actin-fascin array or bundle is actin, this system may provide an ideal system to study the actin filament at a higher resolution without much ambiguity due to the interference from the actin binding proteins as in other systems with scruin or S1.