Antisera to two herpes simplex virus-type 1 (HSV-1) immediate-early gene products ( IE12 and IE175 ) have been produced using oligopeptides, constructed on the basis of proposed DNA coding sequences, as immunogens. In both cases the synthetic peptides were linked to bovine serum albumin via an N-terminal tyrosine prior to immunization. Both the IE12 and the IE175 antisera reacted with their respective HSV-1 antigens and with antigens produced by the HSV-1 immediate-early mutant tsK but not with functionally equivalent antigens induced by HSV-2. IE12 induced by tsK was found to have an altered mobility with respect to the wild-type IE12 and its precipitation was accompanied by a second, minor, component of lower mobility. Revertants of tsK gave similar results. Labelling IE12 with a variety of amino acids indicated that the first of three possible initiation codons was used in its translation from mRNA. The results imply that the other initiation codons were not used. Wildtype IE12 is produced for at least 3 h after release of cycloheximide block and appears to be turned over rapidly.