Intercellular adhesion molecule-1 (ICAM-1) plays an essential role in lymphocyte adhesion to endothelium and migration across endothelial cell barriers. We undertook this study to determine the growth of a murine fibrosarcoma transfected with the ICAM-1 gene. MCA-105 tumor cells were cotransfected with ICAM-1 and the plasmid for neomycin resistance (NeoR). Selected G418-resistant clones were expanded and cell surface ICAM-1 expression was verified using a fluorescence-activated cell sorter. Integration of the ICAM-1 gene and ICAM-1 mRNA expression were verified by Southern and Northern blot hybridization analysis, respectively. C57BL/6 mice were divided into five groups (six animals/group): Control, NeoR only, ICAM- 1 (low expressing, Clone 25), ICAM-1 (high expressing, Clone 81), and a 1:1 mixture of NeoR:Clone 81; animals received 1 x 106 cells on Day 0 and tumor measurements began on Day 7 and were measured in mm2. At 19 days, tumors from cell lines expressing ICAM-1 were significantly (P <.05) smaller than both the parental cell line and tumor-containing NeoR only (364 mm2 vs 466 mm2 and 527 mm2, respectively). This decrease in tumor growth may be a result of increased lymphocyte migration or increased anti-tumor cytotoxicity by infiltrating lymphocytes. The results from the mixed tumor experiment suggest a possible paracrine effect by cells expressing ICAM-1. Studies are currently under way to investigate the effect of immunotherapy on tumors derived from ICAM-1-cloned transfectants.