IE1 of Autographa californica multicapsid nuclear polyhedrosis virus acts as a transactivator of several viral promoters in insect cells. Transient expression assays indicate that IE1 is involved in the activation of the early promoter he65 in both insect TN-368 and mammalian BHK-21 cell lines. IE1 activation of the he65 promoter was compared with IE1 activation of the early 39K promoter. In contrast to the he65 promoter, the 39K promoter was not inducible by IE1 in BHK-21 cells.