The macrocyclic antibiotic rifamycin B was found to be highly surface-active and to aggregate in aqueous solution. The aggregational behavior was studied using small-angle neutron scattering (SANS), Aqueous solutions of rifamycin B showed pronounced scattering at both small-length scales (Q greater than or equal to 0.1 Angstrom(-1)) and at large-length scales ( Q less than or equal to 0.1 Angstrom(-1)). The larger association colloids appear to be rather open low-density aggregates. The addition of 10% 2-propanol greatly reduces the number and size of the aggregates. Somewhat higher amounts of alcohol appear to completely suppress or eliminate aggregation. The suppression of aggregation coincides with the appearance and enhancement of enantioselective association between rifamycin B and a variety of chiral amino alcohols, It appears that the self-aggregation of rifamycin B may be a factor that controls its ability to differentiate between enantiomers in aqueous and hydro-organic solutions.