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Sperm of the brown alga Fucus serratus are highly differentiated, biflagellate, naked cells. Immunolocalisation studies, employing monoclonal antibodies (MAbs - designated FS1 to FS12) raised against antigens of these sperm cells, have revealed that some sperm surface components are distributed over the entire cell, whereas others are restricted to, or occur preferentially on, the surface of the anterior flagellum or cell body. This report describes the use of these MAbs in Western-blot procedures and antigen-modification binding assays to determine the nature of these sperm surface components. Monoclonal antibodies which bind to antigens found on the cell body and both flagella (FS3, FS4, FS6, FS8, FS10) recognise carbohydrate epitopes of a high-molecular-weight glycoprotein (Mr=205 kDa). These MAbs were initially chosen at random from a much larger number of antibodies which bound to sperm in a similar fashion, indicating that this glycoprotein is an immunodominant antigen. Though these MAbs compete under conditions of limited antigen availability, differences in the effects of periodate on antibody binding and differences in other binding data indicate that the MAbs recognise epitopes of this glycoprotein which are neighbouring or overlapping, rather than common. The MAb FS9, which has a similar binding pattern to the above antibodies, also seems to bind to carbohydrate epitopes, but the antigen recognised by this antibody could not be identified in Western-blotting procedures. The MAbs FS7 and FS12, which bind to the mastigonemes on the anterior flagellum and to the cell body and posterior flagellum, recognise a set of glycoproteins in the molecular-weight range 40-250 kDa. The evidence indicates that the antibodies are binding to N-linked carbohydrate side chains of these glycoproteins. Three MAbs that bind to the anterior flagellum (FS2, FS5 and FS11) recognise protein antigens in the molecular-weight range 90-250 kDa; it is not known whether these antigens are glycosylated. The MAb FS1, which binds primarily to the sperm cell body, could not be used in enzyme-linked immunosorbent assays or Western-blotting procedures and the antigen recognised by this antibody is so far uncharacterised.

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