The mechanisms by which cells extend motile pseudopodial projections are still poorly understood. Several fundamental mechanisms have been proposed on the basis of hydrostatic pressure, membrane addition and microfilament reorganization. A common focus of all such mechanisms is the growing tip of a pseudopodium. Yet some basic questions about the nature of the tip in natural pseudopodia remain obscure. However, one class of structure, the virus-tipped projections, often contains a well-defined particle, both morphologically and biochemically, and therefore provides a useful model system for the examination of the tips of cellular projections. In P815 cells the virus-tipped projections are long, thin structures closely resembling filopodia in other cells. The apical virus particle is a retrovirus particle produced by the chronic infection existing in this cell line. In demembranated filopodia, the virus particle retains a tight association with a single actin microfilament. Biochemical analyses indicate that the major retroviral structural polypeptide Pr65 is an actin-binding protein that could provide the anchorage site for the actin filament. The existence of a solid virus particle tethered by an actin filament to the cytoskeleton makes it very unlikely that these projections grow by membrane addition at the tip. The major positive implication is that the apex of a projection does not relinquish its interaction with the submembranous cytoskeleton during growth. Such an arrangement would be compatible with either a hydrostatic-pressure-driven or a cytoskeleton-driven mechanism of filopodial growth.