The presence of H.L.A. antigens in bipolar affective patients attending in the University Hospital of Zaragoza was studied and contrasted with findings in the general population. A random sample of 36 patients was selected for the study. All of them fulfill, both, DSM-III and RDC diagnostic criteria . The HLA antigen was studied by means of Lymphotoxicity Test of N.I.H. standard technique. Both, A29 and B21 HLA antigens were detect in the bipolar patients more frequently that in the controls (Histocompatibility Testing data, 1980), and the differences were statistically significant. It is also suggested that the A2 HLA antigen is more frequently found in bipolar patients "non-respondent" to lithium treatment. The inclination to relate these antigens to specifics genes in the bipolar illness should be pondered, since the risk of "false positives" inherent in studies testing association hypothesis is well known.