Anemia-inducing factor (AIF) was isolated from gastric cancer tissue; however, the human placenta used as the volume of AIF for further analysis did not prove sufficient. This substance was named placental anemia-inducing factor (PAIF). PAIF directly reduces the number of erythrocytes in vitro and reduces the RBC count in rabbits to 80% when i.v. administration of 27 microg/kg of body weight is given. The aim of this study is to better define PAIF and to examine whether the identifical substance expresses on either the surface or in the cytoplasm of established gastric cancer cell lines. PAIF is a glycoprotein with about 20 KD, whose 17 amino acid residues of N terminus were sequenced after Edman treatment. The N-terminus of PAIF were determined as Lqcyncpnptadcktav. This is homologous with that of CD59, which is thought as a regulator of membrane attack complex of complement system. Expression of PAF or CD59 in four established gastric cancer cell lines were examined by indirect immunofluorescence method and by Northern blot hybridization. The cells (1 x 106) were seeded into plastic plates for three days and reacted overnight at 4 degrees C in 0.5 ml of PBS with anti-PAIF polyclonal antibody or with anti CD59 rat monoclonal antibody. Both PAIF and CD59 were stained positively on the surface and/or in the cyroplasm. The total RNAs were prepared from the four kinds of cell lines and normal human lymphocytes. CD59 mRNA was probed in all cell lines by BamH1-EcoR1 fragment of PSRa CD59. The signal levels of MKN-28, MKN-45 and KATO-III were stronger than that of MKN-74, whereas the signal of normal lymphocytes was the lowest. Although there is no decisive evidence that PAIF is exactly the same substance as CD59, and although the biological functions of these two substances are conflictive, and still to be further investigated, the 17 amino acid residues of N-terminus of PAIF expressed in gastric cancer cells were homologous with those of CD59. A derivative of CD59 may exist in gastric cancer.