Sialic acid-binding lectin (SBL-C) from Rana catesbeiana eggs inhibits the growth of tumor cells such as P388 and L1210 leukemia cells (K. Nitta et at, Cancer Res., 54: 920-927, 1994). Here we report the establishment of an SBL-resistant P388 variant cell line, RC-150. Both P388 and RC-150 cells were agglutinated by SBL-C; however, growth of RC-150 cells was unaffected by SBL-C. Cytoplasmic free Ca2+ concentration and transglutaminase activity of RC-150 cells were 0.5 (110 nM) and 3 times (0.62 nmol/mg/min) as high as those of P388 cells, respectively. Microvilli and microplicae were observed on the surface of P388 cells by scanning electron microscopy but were rarely seen on RC-150 cells. Dansylcadaverine-labeled SBL-C bound to both P388 and RC-150 cells. Binding of SBL-C to these tumor cells appears to be mediated by two species of wheat germ agglutinin-stained cell membrane sialoglycoproteins. Labeled SBL-C entered P388 but not RC-150 cells, suggesting that internalized SBL-C acts as an inhibitor of cell proliferation.