ICP4 and ICP0 are two immediate-early proteins of herpes simplex virus type 1 associated with the transcriptional activation of viral genes within infected cells. Previous studies in our laboratory have shown that approximately 100-200 molecules each of ICP4 and ICP0 are associated with the tegument region of virions purified from Vero and HEp-2 cells (F. Yao and R. J. Courtney, 1989, J. Virol. 63, 3338-3344 and 1992, J. Virol. 66, 2709-2716). However, other studies have shown that ICP4 molecules are primarily associated with non-capsid-containing light (L) particles obtained from BHK-21 cells and purified by 5-15% continuous Ficoll gradients (J. McLauchlan and F. J. Rixon, 1992, J. Gen. Virol. 73, 269-276). To reconcile these findings with regard to the association of immediate-early protein with virions obtained from different cell lines, our studies have focused on determining if the host cell type influences the amounts of ICP4 and ICP0 associated with the capsid-containing heavy (H) virus particles (purified virions) compared to the amounts associated with L particles. Virus particles were purified from three different cell lines, Vero, HEp-2, and BHK-21 cells. The H and L virus particles were resolved on 5-15% Ficoll gradients. The results presented indicate that although ICP4 and ICP0 are readily detectable in H particles obtained from Vero and HEp-2 cells, minimal amounts of ICP4 and ICP0 are associated with H particles obtained from BHK-21 cells. The data suggest that the host cell may influence the relative amounts of ICP4 and ICP0 associated with the tegument region of virus particles. Finally, physical particle counts of the H and L particles also suggest that the host cell influences the relative number of L particles produced within the cell.