We studied the diffusion of roxithromycin in the cervix mucus of fifteen healthy normal women, aged 24 to 44 (median 37). They were consulting physician to have an IUD. After this intervention (between the 4 to 7 days of menstrual cycle) they received an antibiotic treatment with the standard dose of roxithromycin: 150 mg bd for a week. At the end of this treatment the cervix mucus was taken 1 to 12 hours after the last antibiotic dose. We dose the roxithromycin by a microbial assay (Bacillus subtilis ATCC 6633, antibiotic medium 1, pH 8) and the acid alpha-1-glycoprotein by an immunodiffusion assay. All women had drug measurable with levels from 0.45 to 2.07 mg/l (median: 0.80 mg/l). The acid alpha-1-glycoprotein levels were quite constant (median: 0.19 mg/l). The antibiotic concentrations observed are above the MIC of the major genital pathogens, mainly C. trachomatis, G. vaginalis, H. ducreyi and U. urealyticum, but lower than the MIC ov N. gonorrhoeae and M. hominis.