Antigens isolated from herpes simplex virus type 1, herpes simplex virus type 2 or cytomegalovirus-transformed hamster cells were tested against 66 sera from noncancer individuals or patients with different types of cancer. By use of the microcomplement fixation procedure to quantify all antigen-antibody interactions, it was observed that 94% (P < 0.001) of all sera from patients with squamous cell carcinoma reacted with antigens from herpes simplex virus type 1-transformed cells, while 84% (P < 0.001) of the same sera reacted with antigen preparations from herpes simplex virus type 2-transformed cells. When sera from patients with adenocarcinoma, sarcoma, liposarcoma and melanoma were tested against these antigens, there was no significant difference in their reactivity from sera of noncancer patients. When sera from all individuals (normal and cancer) were tested against antigens from cytomegalovirus-transformed cells, no significant reaction pattern developed. These studies are the first to describe the isolation of a reactive tumor-associated protein from herpes simplex virus-transformed cells.