AIMS--To compare the recognition of Toxoplasma gondii tachyzoites and cysts by sera, from 10 patients. METHODS--Recognition of antigens from purified tachyzoites (RH strain) and bradyzoites (18691 strain) was compared using western immunoblotting. Sequential serum samples from 10 patients and one laboratory acquired RH infection were used. RESULTS--Recognition of cyst antigens was relatively low and occurred late in infection. The two stages were antigenically distinct with only a few shared bands. CONCLUSION--Immunological recognition of the cystic stage of T gondii is low. This implies that either cysts are poorly immunogenic or that cyst antigen is not available for processing and presentation.