This study was undertaken because of the infrequency of infections due to Candida species in patients with cystic fibrosis despite their extensive treatment with broad-spectrum antibiotics. In vitro susceptibility studies revealed significant inhibition of 11 strains of fungi known to infect human beings by 10 strains of Pseudomonas aeruginosa and nine strains of Pseudomonas cepacia isolated from the sputum of patients with cystic fibrosis. The fungi were Candida krusei, Candida keyfr, Candida guillermondii, Candida tropicalis, Candida lusitaniae, Candida parapsilosis, Candida pseudotropicalis, Candida albicans, Torulopsis glabrata, Saccharomyces cerevisiae and Aspergillus fumigatus. Inhibition of fungal growth by Escherichia coli (NCTC 10418), Staphylococcus aureus (NCTC 6571) and Haemophilus influenzae (NCTC 11931) could not be demonstrated. The continued presence in the sputum of patients with cystic fibrosis of strains of P. aeruginosa and P. cepacia, which produce antifungal substances, may inhibit growth of Candida species and so prevent overt Candida infections. A. fumigatus would seem to be the most important fungus causing disease in patients with cystic fibrosis. It is therefore interesting to note that this was the most resistant of all the fungi tested for inhibition by P. aeruginosa and P. cepacia.