Numerous discrete lesions, which we have termed blebs, appeared in the nucleus of BHK cells 10 to 15 h after infection with herpes simplex virus type 1 (HSV-1). They were formed in the inner portion of the nuclear envelope by the apposition of two thickened lamellae overlying a vacuole. As demonstrated by electron microscopic studies, blebs were regular and associated with the peripheral lamina in the nucleus, averaging 3.5 blebs per micron2. They appeared to be associated with an enrichment of the 155K major capsid protein in the nuclear membrane subfractions as compared with the protein composition of nuclei and plasma membrane fractions. We propose that blebs represent the site of assembly of capsid proteins before DNA insertion and eventual envelopment.