Efflux pumps of the resistance nodulation cell division (RND) transporter family, such as AcrB of Escherichia coli, play an important role in the development of multidrug resistance, but the molecular basis for their substrate promiscuity is not yet completely understood. From a collection of highly clarithromycin-resistant AcrB periplasmic domain mutants derived from in vitro random mutagenesis, we identified variants with an unusually altered drug resistance pattern characterized by increased susceptibility to many drugs of lower molecular weight, including fluoroquinolones, tetracyclines, and oxazolidinones, but unchanged or increased resistance to drugs of higher molecular weight, including macrolides. Sequencing of 14 such "divergent resistance" phenotype mutants and 15 control mutants showed that this unusual phenotype was associated with mutations at residues I38 and I671 predominantly to phenylalanine and threonine, respectively, both conferring a similar susceptibility pattern. Reconstructed I38F and I671T single mutants as well as an engineered I38F I671T double mutant with proved efflux competence revealed an equivalent phenotype with enhanced or unchanged resistance to many large AcrB substrates but increased susceptibility to several lower-molecular-weight drugs known to bind within the distal binding pocket. The two isoleucines located in close vicinity to each other in the lower porter domain of AcrB beneath the bottom of the proximal binding pocket may be part of a preferential small-drug entrance pathway that is compromised by the mutations. This finding supports recent indications of distinct entrance channels used by compounds with different physicochemical properties, of which molecular size appears to play a prominent role.