Serum agglutinins or lectins are reported to be induced in marine molluscs by exposure and may enhance bacterial clearance from the host; however, there is little information on possible relationships between lectins and protozoan parasites of molluscs. Two protozoans, Haplosporidium nelsoni and Perkinsus marinus, cause mortality of eastern oysters, Crassostrea virginica. The authors tested the hypothesis that if hemolymph agglutinins or other hemolymph proteins are important in the defense against these parasites, oysters with high 'baseline' (pre-exposure) levels, or oysters that produce these substances after challenge, should have lower parasite burdens and survive longer than animals without these characteristics. In May 1990, individually labelled oysters were placed in Chesapeake Bay, MD, where they were exposed primarily to P. marinus, and in Delaware Bay, NJ, where they were exposed to both parasites. The authors conclude that the serum agglutinins tested play no role in defense against either H. nelsoni or P. marinus, and that differences in total protein were related to pathology rather than disease resistance.