Abstract
Fourteen multiresistant and 8 ampicillin- or tetracycline-resistant H. influenzae isolates and 4 ampicillin-resistant H. parainfluenzae isolates for plasmid DNA. Sixteen strains carried plasmids. Both plasmid-free and plasmid-carrying isolates transferred the antibiotic resistance by conjugation. All transconjugants carried plasmid DNA, suggesting that the apparent plasmid-free strains contained R plasmids encoding for antibiotic resistance.