Studies were conducted on an adventitious agent (Hz-lv) isolated from the IMC-Hz-1 cell line. It appeared identical to the virus first obtained by Granados et al. from a persistent infection of this cell line. Restriction endonuclease digestion of Hz-lv DNA indicated the agent was different from the S nuclear polyhedrosis virus (HzSNPV) of the host species, Heliothis zea, from which the IMC-Hz-1 cell line was derived by Hink and Ignoffo (W.F. Hink and C.M. Ignoffo, 1970, Exp. Cell Res. 60, 307-309). Hz-1V caused extensive cytopathic effects (CPE) in cultures of TN-368 cells and was moderately infectious to several other insect cell lines. The lytic nature of the infection permitted development of a plaque assay employing TN-368 cells.