Predominant Bifidobacterium strains belonging to the intestinal flora of four human volunteers were isolated on selective medium before and after eight days of treatment with oral amoxicillin-clavulanic acid (Augmentin). These antimicrobial agents are known to be strongly active against the genus Bifidobacterium. A fifth volunteer did not receive the antibiotics and was considered as a control. Bifidobacteria were characterized by hybridizing a ribosomal 23S DNA probe onto their EcoRV restriction patterns, and were identified by comparing the ribosomal patterns obtained to collection strains. A total of 17 distinct ribosomal patterns and 23 distinct pattern types were revealed for the 95 isolates tested. Each type characterized was correlated with a specific ribosomal pattern associated with a specific total restriction pattern. Similar-sized molecular bands permitted isolates to be unambiguously discriminated into the species B. longum, B. bifidum, and B. adolescentis. This study enabled us to show considerable strain variability among individuals. Three months after penicillin ingestion, no significant changes were observed in Bifidobacterium flora. Each flora remained relatively stable for strain composition over time, with some slight variations also detected in our control subject.