An indirect fluorescent antibody test (IFAT) incorporating a polyclona
l antibody to Marteilia sydneyi recognized sporulating stages of M. sy
dneyi from Saccostrea commercialis but not those of Marteilia refringe
ns, M. maurini, Marteilia sp. and Marteilioides branchialis from Ostre
a edulis, Mytilus galloprovincialis, Mytilus edulis and Saccostrea com
mercialis respectively. This indicates that the antibody had a high sp
ecificity and that the other parasites were immunologically distinct f
rom M. sydneyi. Immunoelectron microscopy was used to investigate back
ground labelling and the specificity of the antibody to antigenic site
s. It showed that though most immunoglobulins were specific to parasit
e epitopes, some reacted to host tissue. IFAT's based on three monoclo
nal antibodies raised against Marteilia sp. did not recognise spores o
r other stages of M. sydneyi. An immunogold-silver staining technique
using the polyclonal antibody to M. sydneyi failed to identify the pre
sumed presporulation stage of M. sydneyi in the connective tissue of a
recently infected host. This suggests the antigens were stage-specifi
c. Thus a DNA probe rather than immunohistochemical tests may be more
useful in investigating the life cycle of this parasite.