Mucus and fluid from the pallial (mantle) cavity of eastern oysters Crassostrea virginica Gmelin from Chesapeake Bay and Galveston Bay were found to agglutinate a variety of vertebrate erythrocytes (RBC) and bacteria. Agglutinating activity of pallial cavity fluid was probably due to dissociation of agglutinins from mucus on the external surfaces of organs. Agglutination titers of pallial cavity fluid from individual Galveston Bay oysters for six different RBC were positively correlated with high significance, indicating a strong interdependence regardless of RBC specificity. The relative agglutinating activity for different RBC by tissue explants and homogenized tissues (mantle, gill, digestive gland and adductor muscle) was similar to the relative activity of the pallial cavity fluid; mantle and gill tissue had the greatest agglutinating capacity over all RBC, whereas the adductor muscle had the least. Positive correlation was detected between agglutination titers of pallial cavity fluid (for different RBC) and titers of hemolymph agglutinins from the same oysters. It is hypothesized that agglutinins in the pallial cavity and hemolymph have a common source. The results of the explant and homogenate assays can be interpreted to suggest that the source is mantle and gill tissues. (Copyright (c) 1992 Elsevier Science Publishers BV.)