This study characterizes humoral opsonins from the tunicate, Pyura stolonifera. The predominant opsonic components in P. stolonifera hemolymph were found to be calcium-dependent lectins with broad carbohydrate specificities. The opsonic lectins were purified by carbohydrate affinity chromatography which eluted a complex pattern of proteins ranging in molecular mass from 80 to >200kDa. Reducing and two dimensional SDS-PAGE indicated that the diversity of mature lectins evident under non-reducing conditions resulted from the differential oligomerization of two polypeptide sub-units (35 and 22kDa). In addition to lectin-mediated opsonic activity, hemolymph was also found to contain proteolytically activated opsonins. These data suggest that multiple, possibly interactive opsonic systems co-exist in P. stolonifera.