The influence of clindamycin, dicloxacillin, minocycline and norfloxacin on the faecal concentration of urobilinogen was investigated. The studied drugs were administered orally in standard dosage for six days to groups of six volunteers. A decrease in faecal concentration of urobilinogen following administration of clindamycin (P<0.01) and="" dicloxacillin=""><0.05) was="" found.="" the="" possible="" predictive="" value="" of="" a="" decrease="" of="" the="" faecal="" level="" of="" urobilinogen="" as="" an="" indicator="" for="" the="" impairment="" of="" microbial="" colonization="" resistance="" and="" for="" the="" risk="" of="" failure="" of="" oral="" anticonceptive="" treatment="" is="" discussed.="" it="" is="" suggested="" that="" clindamycin="" and="" dicloxacillin="" should="" not="" be="" combined="" with="" oral="" anticonceptive="" treatment="" unless="" more="" specific="" investigations="" have="" excluded="" interaction="" of="" these="" drugs="" with="" the="" oestrogen="" metabolism="" in="" the="">