Antitoxoplasma globulin was labelled with fluorescein and was used to stain Toxoplasma gondii organisms in smears of peritoneal exudate. The evidence indicates that this staining was due to an antigen-antibody reaction at the cellular level. Methods are presented for handling the organisms so that they can be stained over a period of several months. Some general characteristics of the antigen system involved are described.

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