Pre- and postimmunization sera from eight tumor-free melanoma patients undergoing vaccinia melanoma oncolysate (VMO) therapy were used to investigate the humoral response to antigens from infected and uninfected melanoma cells and from vaccinia virus. Immunodetection on Western blots showed that all patients, in addition to reacting to several other proteins, developed IgG antibodies to a M(r) 31,000 protein antigen within 1 month of immunization. This M(r) 31,000 antigen is expressed both on VMO and on melanoma metastases in situ, disappears in primary cultures of these metastases, and is absent in extracts from vaccinia virus, from human melanoma cell lines, and from normal melanocytes, suggesting that this M(r) 31,000 protein is reexpressed following vaccinia virus infection of human melanoma cells. Periodate treatment of the blotted antigens abolished reactivity of patients' postimmunization sera with the M(r) 31,000 antigen, thus showing that this antigen is a glycoprotein and that the relevant epitope is likely to reside on its carbohydrate moiety. These anti-M(r) 31,000 IgG antibodies were absent in the sera of VMO-treated patients before immunization, absent in the serum of a normal donor hyperimmunized with vaccinia virus, and absent in normal human sera. In addition, these anti-M(r) 31,000 antibodies appeared 1 week after the first VMO injection, remained stable during the treatment, and decreased when the treatment was stopped. Such antibodies can also be demonstrated in sera of melanoma patients bearing metastases but disappeared following resection of their metastases. Thus, in melanoma patients, immunization with VMO induces an antibody response directed against a M(r) 31,000 glycoprotein likely to represent a new melanoma antigen. Further identification of this antigen could be of utmost interest for the further development of melanoma vaccines.