Quantitative binding studies of [G-3H]streptogramin A and [G-3H]streptogramin B with high-salt-washed ribosomes were carried out in the presence of a minimum of 10% (v/v) ethanol because of the antibiotic insolubility in water. It was observed that the presence of streptogramin A increases the affinity of [G-3H]streptogramin B for the ribosome. Thus the dissociation constant for [G-3H]-streptogramin B interaction with the ribosome is Kd=13.3 nM in the presence of streptogramin A and Kd=59 nM in its absence. Furthermore the values for the dissociation constants for [G-3H]-streptogramin B interaction in the presence of 50% (v/v) ethanol, were Kd=0.13 micronM in the presence of streptogramin A and Kd=0.70 micronM in its absence. This increased affinity of [G-3H]streptogramin B in the presence of streptogramin A can explain the synergistic effects of mixtures of streptogramins A and B at the ribosome level.