Both monensin and cytochalasin D reduced the production of infectious cell associated virus and infectious extracellular virus with the latter clearly being the more sensitive. The difference in yields were even more clearly seen if the yield of virus particles was monitored instead of yield of infectious virus. Addition of 1 microgram/ml cytochalasin D or 1 microM monensin to the growth medium of vaccinia virus-infected cells inhibited the appearance of extracellular enveloped vaccinia virus (EEV) in the growth medium without affecting the production of intracellular naked vaccinia virus (INV) particles. Although EEV was not released into the medium of cytochalasin D treated cells, EEV was, nevertheless, detectable in CsCl gradients of cell associated virus. Monensin treatment did not affect the synthesis of vaccinia glycoproteins but did significantly reduce the transport of these glycoproteins to the cell surface and also reduced the secretion of proteins. Monensin had the additional effect of causing the appearance of numerous large vacuoles in the cytoplasm. Vaccinia is normally wrapped by cytoplasmic membranes in preparation for release. The monensin induced conversion of cytoplasmic membranes to large vacuoles is presumably the basis for the block in virus wrapping and subsequent release. Cytochalasin D did not alter any of the steps in protein synthesis, transport or secretion. Electronmicroscopic studies confirmed the existence of EEV on the surface of infected cells treated with cytochalasin D. This drug which specifically affects cellular microfilament organisation thus imposes a block on the final release of EEV from the cell surface.