Previous studies (R. J. Watson and J. B. Clements, Virology 91:364--379, 1978; C. M. Preston, J. Virol. 29:275--284, 1979) have shown that the herpes simplex virus type 1 (HSV-1) mutant tsK has a temperature-sensitive lesion in an immediate early polypeptide whose function is to induce synthesis of new viral transcripts, including mRNA, for pyrimidine deoxyribonucleoside kinase. The studies presented here examine the properties of immediate early polypeptides in wild-type HSV-1 and tsK-infected cells at 31 and 38.5 degrees C. The overall pattern of immediate early protein synthesis was similar in wild-type HSV-1- and tsK-infected cells when radiolabeled with [35S]methionine or 14C-amino acid mixture. Further investigation, however, revealed two aberrant properties of the polypeptide Vmw 175 in tsK-infected cells at 38.5 degrees C. Upon cell fractionation, large amounts of this polypeptide were recovered in the cytoplasmic fraction, in contrast to tsK-infected cells at 31 degrees C or wild-type HSV-1-infected cells at either temperature. Furthermore, at 38.5 degrees C tsK-induced Vmw 175 was not processed normally to forms of lower electrophoretic mobility. Both of these defects were reversible upon downshift of tsK-infected cells, even in the absence of further protein synthesis, but were not observed in cells infected with a revertant of tsK. Coinfection of tsK-infected cells with wild-type HSV-1 did not alleviate these lesions, suggesting that they resulted from an abnormal Vmw 175 polypeptide rather than from a defective processing enzyme. Temperature upshift of tsK-infected cells caused reversion of Vmw 175 to the mutant form. The progression to synthesis of late polypeptides was also arrested; therefore, a functional lesion was also reversible upon temperature changes between 31 and 38.5 degrees C during the early stages of infection. The identification of a polypeptide with abnormal properties in tsK-infected cells and the demonstration that these properties, and the functional lesion, are reversible may provide an important system for investigation of HSV-1 transcriptional control.