To explore the intrafamilial homology and longitudinal stability of colonization by early acquired mutans streptococci, genomic fingerprinting was performed on 254 strains (192 Streptococcus mutans and 62 Streptococcus sobrinus strains) collected from 16 families (16 mother-child pairs, seven fathers and four siblings). Genomic DNA was digested by the restriction endonuclease HindIII, followed by gel electrophoresis, Southern blotting, and hybridization with a digoxigenin-labeled 16S rDNA probe, and hybrid detection by enhanced chemiluminescence. One to five ribotypes were identified per person, and between two and nine (median five) within each family. Fourteen of the 16 mother-child pairs showed homology for at least one ribotype (range 1-4). Six of the seven father-child pairs had one ribotype in common. Ten of the 13 longitudinally examined children showed persistence of at least one ribotype over a period of up to 16 yr. The results support the notion of intrafamilial transfer of mutans streptococci, and suggest that colonization of early acquired strains persists into young adulthood.