A permanent human tumor-line PC-93 was HLA allotyped with the use of a complement-dependent cytotoxicity test supplemented with absorption experiments. In comparison with the peripheral blood lymphocytes of the patient, whose adenocarcinoma of the prostate was the origin of PC-93, remarkable differences were observed. The tumor line lacked the expression of the genetically appropriate HLA-A1, A2 and B15 antigens. Instead of these antigens, cross-reactive HLA antigens seemed to be expressed on PC-93, i.e. All, A28 and B17. Absorption experiments showed that the antibodies reactive with the neo-HLA antigens were not the same as those reactive with the specific HLA antigens. This alien expression of HLA-like structures was specific for the patients' prostatic tumor line, as EB-virus transformed B-lymphocytes of the patient expressed the genetically appropriate HLA antigens.