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Cells of the yeast phase of the dimorphic systemic fungus pathogen Histoplasma capsulatum readily released large numbers of intact protoplasts without degradation of their cell walls by snail or microbial enzymes, previously regarded as a requirement for all yeast and mycelial fungal forms. Over 90 percent of "B" type yeast cells in the early logarithmic phase of growth released living protoplasts when incubated at 37 degrees C with 2 molar magnesium sulfate, whereas "A" type yeast cells required prior exposure for 24 hours to 2-deoxy-D-glucose before incubation in the 2 molar magnesium sulfate.

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A colorimetric method using a tetrazolium compound, 3,4,5-dimethylthiozalil-(1 or 2),2,5-diphenyltetrazolium bromide (TTBr), was developed for studying the growth activity of yeast-phase Histoplasma capsulatum. Materials extracted in phosphate buffer, pH 7.0, from cells at different stages of growth reduced ITBr. Colorimetric changes were correlated with enzymatic activity. Under standardized conditions specified herein, the optical density of the reduced tetrazole was an index of the growth activity of the organism.

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