A disease of eastern oysters, Crassostrea virginica (Gmelin, 1791), caused by a protistan, Haplosporidium nelsoni, has caused great losses in the oyster fisheries of the northern Atlantic coast of North America. Certain oyster stocks have been selectively bred to survive infection by H. nelsoni (often called MSX disease) but mechanisms of resistance are not known and potential resistance to another protistan parasite, Perkinsus marinus, is not clear. Oysters from MSX-resistant stocks and from unselected (susceptible) stocks were compared over one year (1988-89) at an estuarine site in Chesapeake Bay where MSX disease and P. marinus ('dermo' disease) are both known to occur. Prevalence of MSX disease was 0%-4% for the resistant stock and 36%-60% for the susceptible stock, whereas prevalence of P. marinus was similar for both resistant (58%) and susceptible (67%) stocks. Comparison of putative defense mechanisms revealed no differences in hemocyte capacity to spread, respond to salinity changes or locomote in vitro. The susceptible stock exhibited higher serum protein concentrations and higher lysozyme concentrations during spring and summer. Serum agglutination titers for Vibrio cholerae CA401 were consistently higher for the resistant stock during summer; serum lectins could be related to disease resistance or affected by H. nelsoni infection.