Nuclear antigen in Novikoff hepatoma chromatin was partially purified and characterized. As indicated by complement fixation assay, this antigen was present in chromatin of embryonic livers and several transplantable tumors. It was not detected in normal tissue chromatins of the same animals. For its immunological specificity this protein antigen (molecular weight 45,000-60,000) had to be complexed with DNA. Preliminary experiments indicate that specific nuclear protein antigens are also present in human tissues and spontaneous malignancies.