African green monkey kidney cells (Cos7) were cultured in Dulbecco's modified Eagle's medium (DMEM) (Invitrogen, Germany) supplemented with 10% fetal calf serum (FCS), glutamine (100 mg/l) and gentamicin (350 mg/l). HCMV was propagated in human fetal lung fibroblasts (MRC-5) or human foreskin fibroblasts (HFF) grown in DMEM supplemented with 10% FCS, glutamine and gentamicin as above. For immunofluorescence cells were grown on 13-mm glass coverslips in 24-well plates. HB15-UL84prluc represents a recombinant AD169-based HCMV which expresses the firefly luciferase gene under the control of the HCMV UL84 promoter.