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Delayed-type hypersensitivity develops late in the course of human toxoplasmosis, and a positive skin test is of some value for implicating chronic or eliminating acute forms of toxoplasmosis as a cause of disease. Toxoplasma-infected guinea pigs were studied to determine the onset and development of delayed-type hypersensitivity. Both the toxoplasmin skin test and the in vitro macrophage migration inhibition technique indicated that delayed hypersensitivity to toxoplasma antigen existed as early as 1 week after infection. The mechanism responsible for the observed inhibition of macrophage migration in vitro appeared to be an inhibitory factor(s) released from sensitized lymphoid cells in the presence of antigen.

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