Emergence of antibiotic resistance is related to the ease of mutation, to the extent of exchange of genetic information in bacteria by conjugation, transformation, and transduction, and to the large-scale use of antimicrobial agents in the biosphere. In addition to the development of resistance through chromosomal mutation and exchange of chromosomal genes among organisms, there is a more profound enlargement of the gene pool by the dissemination and amplification of plasmids. Two examples of the exchange of antibiotic resistance are analyzed: the transfer of plasmids from Bacteroides fragilis to Escherichia coli and the emergence of antibiotic-resistant strains of Streptococcus pneumoniae. Plasmids encoding antibiotic resistance in B. fragilis were transferred to E. coli by DNA-mediated transformation and conjugation. The β-lactamase in the transformants and transconjugants displayed the same substrate specificity and electrophoretic mobility as the donor strain. The plasmid apparently was integrated rapidly into the chromosome of the recipient strain. Multiple antibiotic-resistant strains of S. pneumoniae were analyzed for plasmids, and none were detected. Furthermore, no evidence of linkage between the traits of multiple antibiotic resistance was observed. β-Lactamase was not detected in the penicillin-resistant strains; therefore, it is likely that the resistance in these strains was chromosomal rather than plasmid-mediated. The range of genetic exchange and the use of Koch's postulates in determining the genetic mechanism of antibiotic resistance are illustrated and discussed.