We used selective media together with aerobic and anaerobic incubation for the quantitation of common pathogens in liquefied sputum from children with cystic fibrosis. The accuracy of the technique was verified by reconstruction studies in which laboratory strains with antibiotic-resistance markers were added to sputum from cystic fibrosis patients. Comparison of the numbers of bacteria found on quantitative culture of clinical specimens with the "predominant" organism found on routine culture yielded a poor correlation. When Pseudomonas aeruginosa was the most prevalent on routine culture, it was present in the highest numbers on quantitative culture (mean count = 10(8) cfu/g). However, large numbers of Haemophilus influenzae (mean count = 10(7) cfu/g), Staphylococcus aureus (mean count = 2 X 10(6) cfu/g), and streptococci (mean count = 2 X 10(6) cfu/g) were also present in these cultures. When S. aureus was the predominant organism, H. influenzae and P. aeruginosa were also present in similar numbers (c. 10(7) cfu/g). When H. influenzae was the predominant species on routine culture, the mean count was 7 X 10(6) cfu/g and P. aeruginosa was often completely absent. We conclude that the selective technique permits reliable enumeration of sputum bacteria, and offers a more accurate assessment of the microbial flora of sputum in cystic fibrosis than does simple plating of unhomogenised sputum.