Ten healthy volunteers, from whom no erythromycin-resistant oral streptococci could be isolated initially, received three doses of erythromycin stearate (1.0, 0.5 and 0.5 g) on two separate occasions with a one-week interval between them. After the second administration all volunteers yielded moderately resistant strains (MIC 1-4 mg/l) and four harboured highly resistant streptococci (MIC 16- greater than 256 mg; MBC 128- greater than 256 mg/l). Erythromycin-resistant strains persisted in eight of the volunteers at 23 weeks and in five of eight volunteers at 43 weeks. Species included Streptococcus sanguis, Str. mitior and unclassified streptococci, and dextran-positive strains were encountered. Erythromycin-resistant streptococci are thus readily selected by two administrations of the three-dose regimen. Until the time of emergence of the resistant strains is further clarified a full assessment of the antibiotic sensitivity of the flora of the gingival sulcus is advisable before erythromycin prophylaxis is repeated.