AlnA is the protein responsible for the emulsifying and solubilizing activity of the Acinetobacter radioresistens KA53 bioemulsifier alasan. AlnA was produced in Escherichia coli, purified to homogeneity and then used to measure the enhanced solubility of 12 polyaromatic hydrocarbons (PAHs). The amount of PAH solubilized was directly proportional to AlnA concentration. The ratio of PAH solubilized by 40 mug/ml AlnA compared to that soluble in the aqueous buffer varied greatly, from 4 (fluorene) to 81 (hexylbenzylcyclosilane). Calculations of moles PAH solubilized per mole AlnA yielded values from 4.3 (hexylphenylbenzene) to 55.8 (1,10-phenanthrolene). There was no obvious relationship between the amount of PAH solubilized and its molecular weight or intrinsic solubility. Native gel electrophoresis indicated that AlnA formed hexamers in the presence of PAHs. With molar ratios of fluorene to AlnA of 0.75 or less, only the monomer was observed, whereas at ratios of 7.5 or higher, only the hexamer was detected. At an intermediate molar ratio of 2.6, both monomer and hexamer appeared. The data indicate that PAHs are initially solubilized by binding to the monomeric form of AlnA, and as the amount bound increases above one molecule PAH per AlnA, the protein aggregates to form a specific oligomer of 5-8 monomers which allows for the binding and solubilization of more PAH.