There are relatively few organisms which are known to cause persistent latent infection in man, and which for years produce an antigenic stimulus. Toxoplasma gondii is one such organism, and serologic evidence of infection with this protozoan is present in 20 to 50% of normal adults in the United States. In recent years serious infection with this intracellular parasite has been recognized with increased frequency in patients receiving immunosuppressive therapy (1, 2). The underlying host factors which control this infection, despite persistent activity in its latent form, are not completely understood, but resistance appears to be mainly cell mediated (3).

A skin test elicits delayed hypersensitivity but does not become positive for months or years after the acute infection (4, 5). Lack of standardization and the possibility of adventitious agents in the antigen have prevented the widespread use of this skin test in this country (6).

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