Porifera (sponges) are the oldest extant metazoan phylum. Dissociated sponge cells serve as a classic system to study processes of cell reaggregation. The reaggregation of dissociated cells is mediated by an extracellularly localized aggregation factor (AF), based on heterophilic interactions of the third order; the AF bridges two cells by ligating a cell-surface-bound aggregation receptor (AR). In the present study we report cloning, expression and immunohistochemical localization of a polypeptide from the marine sponge Geodia cydonium, which very likely represents the AR. The presumed AR gene gives rise to at least three forms of alternatively spliced transcripts of 6.5, 4.9 and 3.9 kb, as detected by northern blotting. Two cDNA clones corresponding to the shorter forms were already reported earlier; here we present an analysis of the largest. All three putative polypeptides feature scavenger receptor cysteine-rich (SRCR) domains. The largest form, SRCR-SCR-Car, is a cell-surface receptor of molecular mass 220 kDa, which is assumed to be the cell-adhesion receptor AR; the second form, SRCR-Re, is also a putative receptor of 166 kDa, while the third form, SRCR-Mo, is a soluble molecule of 129 kDa. The SRCR-SCR-Car molecule consists of fourteen SRCR domains, six short consensus repeats (SCRs), a C-terminal transmembrane domain and a cytoplasmic tail; its fourteenth SRCR domain features an Arg-Gly-Asp tripeptide. To obtain monoclonal antibodies, a 170-amino-acid-long polypeptide that is found in all three forms of the SRCR-containing proteins was expressed in E. coli. In a western blot of sponge cells lysate the monoclonal antibody raised against the recombinant polypeptide recognized two major immuno-reacting polypeptides (220 and 117 kDa) and two minor bands (36 and 32 kDa). The antibody was found to react with antigen(s) predominantly localized on the plasma membranes of cells, especially those of spherulous cells. In a functional assay Fab' fragments of the antibodies suppressed AF-mediated cell-cell reaggregation. Additionally, a recombinant SRCR-soluble fragment effectively inhibited AF-mediated cell-cell reaggregation. We conclude that the 220 kDa SRCR-containing protein of the sponge G. cydonium is very likely the AR.