We have investigated the influence of the macrolides erythromycin and josamycin on the selection of resistant oral streptococci by sampling saliva of volunteers before and after oral administration of 1.5 g of either agent followed by a further 0.5 g of the same drug after 6 h, as in routine prophylaxis for oral or dental procedures. The small proportion of resistant organisms present before antibiotic administration increased substantially 48 h after the macrolides were given. After antibiotic administration mean counts of oral streptococci resistant to 1, 4 and 64 mg/l erythromycin were 23%, 17% and 6%, respectively, of the total numbers of streptococci isolated; after josamycin, the values were 13%, 6% and 4% respectively. The proportion of resistant streptococci then declined gradually but remained above pre-antibiotic levels three months later. Streptococci isolated on media containing 64 mg/l of macrolides were mainly Streptococcus sanguis and S. mitis with one isolate of S. salivarius; the majority were resistant to 256 mg/l erythromycin, josamycin, all other macrolides tested and clindamycin. Amoxycillin and pristinamycin were the only compounds tested that were inhibitory to these organisms at therapeutically attainable concentrations.