A survey of fungal aerospora over a 2-year period was combined with an investigation of the fungi cultured from 295 samples of peripheral human lung and 2466 samples of sputum over the same period. 83% of lung and 88% of sputum samples contained culturable fungi, although in 16% of lung and 31% of sputum samples Candida albicans was the only one. Those fungi of small spore size tended to be found in lung tissue while those with larger spores were more likely to be found in the sputum. While their presence in the respiratory tract was related to their occurrence in the air, differences in their frequencies in the air and lung suggested that the lung had a selective inhibitory effect on the development of different spores. In particular, Aspergillus fumigatus was cultured more frequently than would have been anticipated from its profusion in the air.