Actin the main component of the cellular microfilament network, is present in human respiratory syncytial virus (HRSV) purified virions, as an internal component. This fact and the results of immunoprecipitation studies indicate that during HRSV infection in HEp-2 cells there are interactions between cellular actin and viral components, that can promote a transitory increase in the polymerization of synthetized actin, mainly of the beta isotype. This increased actin polymerization can be related with the formation of cytoplasmic extensions, that contain beta actin and viral particles observed in the HRSV infected HEp-2 cells. The formation of these structures may indicate that HRSV has developed an actin-based motility system similar to that described for other viral and bacterial systems.