Combining aqueous two-phase partitioning and PEG fractional precipitation, a simple and effective purification process for the target intra-cellular enzyme beta-galactosidase (beta-gal) from E. coli (W 3110) cells was developed based on the evaluated differences between surface properties of beta-gal and those of other total soluble proteins. beta-gal was partitioned to the PEG rich top phase of the used aqueous two -phase system and finally recovered at high selectivity and yield with the following PEG fractional precipitation mediated by the addition of salt to the separated top phase.