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Methenamine silver stain was used to compare the number of cysts of Pneumocystis carinii contained in lung concentrate smears of homogenized lung tissue with the number in impression smears. Results were also compared with histopathological examination of methenamine silver-stained paraffin-embedded sections. Of slides from 23 preparations, a greater number of cysts were contained in concentrate smears than in impressions (P < 0.001). In four preparations, cysts were noted in concentrate smears only. All concentrate smears were positive, whereas 11 of the 23 histopathological sections were negative (P < 0.01). The ability to detect the cyst phase of P. carinii in lung tissue is enhanced by the use of concentrate smears.

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