Understanding of the adhesion molecules involved in the interaction of gastric cancer cells with peritoneal mesothelial cells (PMC) might provide a useful strategy for interfering with cancer cell implantation to the peritoneum. Since CD44H is a major cell surface receptor for hyaluronate (HA), it may play an important role in mediating cancer cell adhesion to PMC through recognition of mesothelium associated HA. The purpose of the present study was to define the functional significance of CD44H in peritoneal metastasis of gastric cancer cells by using quantitative assays for measuring the adhesion ability to PMC in vitro. CD44 expression in gastric cancer cell lines SNU-1, -5, and -16 were examined by reverse transcription-polymerase chain reaction (RT-PCR) and Western blot analysis. CD44H was expressed in SNU-5 and -16 whereas no significant level of CD44H was expressed in SNU-1. SNU-16 expressed additional high-molecular-weight CD44 isoforms. In the adhesion assay with HA coated microtiter wells, SNU-5 and -16 demonstrated high affinity for HA, with values of 64.6+/-10.5% and 91.1+/-8.8%, respectively. SNU-1 demonstrated no significant binding affinity for HA (p<0.05). For the in vitro binding assay, we used a confluent monolayer of PMC. 2.5x10(4) and 5x10(4) gastric cancer cells labelled with 51 Cr were inoculated into microtiter wells and allowed to adhere for 2 hr. After washing of the unattached cancer cells, adherent cells were lysed with 0.1 N NaOH and the radioactivity was measured in the lysates. SNU-5 and -16 bound to PMC, to the extent of 10.0+/-1.0%, 14.6+/-1.8% and 22.9+/-2.0%, 23.7+/-3.6%, respectively. In contrast, SNU-1 demonstrated lower binding ability(6.4+/- 0.5%, 6.7+/-1.3%, respectively) to PMC. In conclusion, CD44H is at least partly involved in gastric cancer cell binding to PMC, which mediates an important first step of peritoneal implantation.