An indirect fluorescent antibody test (IFAT) incorporating a polyclonal antibody to Marteilia sydneyi recognized spornlating stages of M. sydneyi from Saccostrea commercialis but not those of Marteiliarefringens, M. maurini, Marteilia sp. and Marteilioides branchialis from Ostrea edulis, Mytilus galloprovincialis, Mytilus edulis and Saccostrea commercialis respectively. This indicates that the antibody had a high specificity and that the other parasites were immunologically distinct from M. sydneyi. Immunoelectron microscopy was used to investigate background labelling and the specificity of the antibody to antigenic sites. It showed that though most immunoglobulins were specific to parasite epitopes, some reacted to host tissue. IFAT's based on three monoclonal antibodies raised against Marteilia sp. did not recognise spores or other stages of M. sydneyi. An immunogold-silver staining technique using the polyclonal antibody to M. sydneyi failed to identify the presumed presporulation stage of M. sydneyi in the connective tissue of a recently infected host. This suggests the antigens were stage-specific. Thus a DNA probe rather than immunohistochemical tests may be more useful in investigating the life cycle of this parasite.