Ceacam6-L (carcinoembryonic antigen-related cell adhesion molecule 6-long) has recently been isolated from rat testis by differential display technique followed by RT-PCR and DNA sequence analysis. CEACAM6-L is a member of the immunoglobulin (Ig) superfamily and composed of an Ig-like domain, three IgCAM domains, a transmembrane region, and a short intracellular domain. We previously reported that CEACAM6-L was localized at the interface between Sertoli cells and elongating/elongated spermatids and might be an adhesion molecule contributing to apical ectoplasmic specialization in testis. In this study, we investigate the adhesive capacities and the complex structures of CEACAM6-L, using COS-7 cells as a study model. Transfection and immunoprecipitation experiments showed that CEACAM6-L expressed in the cells was distributed to the plasma membrane and interacted homophilically between transfected COS-7 cells. A chemical cross-linking experiment and a binding assay with recombinant CEACAM6-L proteins suggested that CEACAM6-L could form trans-tetra complexes, constructed by cis-homodimers at the adhesion site between COS-7 cells expressing CEACAM6-L. We hypothesized that CEACAM6-L expressed in both germ cells and Sertoli cells forms homophilic trans-tetra complexes between these cells in the seminiferous epithelium.