The results of analytical gel filtration chromatography experiments under various conditions showed each of three flavoprotein monohydroxybenzoate hydroxylases, p-hydroxybenzoate 3-hydroxylase, m-hydroxybenzoate 6-hydroxylase, and salicylate 5-hydroxylase, from the gram-positive Rhodococcus erythropolis strain S1 to be present in solution mainly as a tetramer at 4 degrees C and as a dimer at 30 degrees C. In the presence of 1% Tween 80, the three flavoenzymes were each observed to be a monomer with activity about half that of the dimer.