Human cytomegalovirus (HCMV), a ubiquitous beta herpesvirus, is composed of a double strand DNA, coated by a proteinacious capsid, which is surrounded by a tegument layer and an envelope containing glycoproteins. The tegument layer of HCMV contains approximately 30 virus-coded proteins. In this work I studied the function of two viral gene products: pp28, a 190 aminoacid protein encoded by the UL99 open reading frame and UL38, a 331 aminoacid protein encoded by UL38 open reading frame. I have constructed human cytomegalovirus mutants that fail to produce the pp28 and UL38 proteins using a bacterial artificial chromosome system and propagated them by complementation in pp28 and UL38 expressing fibroblasts. Both mutant viruses are profoundly defective for growth in normal fibroblasts, and I demonstrated that both are required for assembly of virus particles. In absence of the UL38 protein, capsids accumulate in the nucleus and fail to move efficiently to the cytoplasm to acquire their final set of tegument proteins and envelope. In the absence of the pp28 protein, capsids associated with tegument proteins accumulate in the cytoplasm, but fail to acquire their final envelope.