Abstract
Summary 1. A pyrimethamine-resistant line of Toxoplasma at least 640 times more resistant than the parent strain was developed in vitro. 2. The resistant nature of this line was retained for at least 3 months when the parasite was grown in the absence of pyrimethamine. 3. A pyrimethamine-requiring line of Toxoplasma could not be developed. 4. The drug-resistant nature of this line of Toxoplasma was not demonstrable in vivo.

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