CD98 is a multifunctional transmembrane protein found on the surface of many activated cell types and is involved in the regulation of cellular differentiation, adhesion, growth, apoptosis as well as amino acid transport, although the exact mechanisms underlying most of these functions remain unknown. It has been reported that some antibodies against CD98 suppress virus-induced cell fusion and CD98-mediated cell fusion of monocytes while others, cross-linking CD98, stimulate cell aggregation and growth, indicating that CD98 molecules are able to regulate cell fusion. CD98 is expressed on placental cytotrophoblast and its involvement in syncytiotrophoblast formation has been suggested (Kudo et al., 2003). We used BeWo cells (a choriocarcinoma cell line) to study the molecular mechanism of syncytialisation in relation to the expression of CD98 and its regulation, before and after stimulation with Forskolin (cAMP treatment), incubation of anti-CD98 antibodies and antisense CD98 oligonucleotides. BeWo cells (unlabelled or labelled with DiO or Mito-tracker deep red) were cultured in six well plates and treated, at 50-60% confluence, with Forskolin (100μM) and with or without the addition of monoclonal CD98 antibodies (mab4F2 and mabAHN18) or antisense oligonucleotide to CD98. The expression and distribution of CD98 and the degree of cell fusion were measured by western blotting, by multiple colour flow cytometry, and by immunocytochemistry and immunoelectron microscopy. CD98 expression on the cell membrane increases by 40% in cells treated with Forskolin compared to control cells (P=0.0007, two-tailed t test), with a concomitant increase in fusion. CD98 expression is reduced to control levels by addition of antisense oligonucleotide. CD98 protein is increased significantly in both control and forskolin treated cells incubated with either monoclonal antibodies (P=0.015 in control cells, P=0.002 in forskolin treated cells, two-tailed t test). Thus flow cytometry, western blotting and confocal and electron microscopy results show recruitment and clustering of CD98 molecules from cytoplasm to cell membrane in response to cross-linking with monoclonal anti-human CD98. These data suggest that recruitment of activated CD98 to the cell membrane may be integral to the normal process of syncytialisation.