The tumor growth and the invasive capacity of a rat glioma cell line (BT4Cn) were studied after transfection with the human transmembrane 140‐kDa isoform of the neural‐cell adhesion molecule, NCAM. After s.c. injection, the NCAM‐transfected cells showed a slower growth rate than the parent cell line (BT4Cn). Upon intracerebral implantation with BT4Cn cells and different clones of NCAM‐transfected cells, all animals developed neurological symptoms within 13‐16 days. However, the tumors showed different growth characteristics. The NCAM‐transfected BT4Cn cells were localized in the region of the injection site, with a sharply demarcated border between the tumor and brain tissue. In contrast, the parental cell line showed single‐cell infiltration and more pronounced destruction of normal brain tissue. Using a 51Cr‐release assay, spleen cells from rats transplanted with BT4Cn tumor cells generally showed a lower cytotoxic response than the spleen cells from rats transplanted with the transfected variants of BT4Cn cells, indicating that the transfection procedure in itself mediated an activation of the immune system. The present data suggest that NCAM may influence the malignant behavior of rat glioma cells in vivo. © 1994 Wiley‐Liss, Inc.