Selected microbial lactases are used to treat lactose intolerance. A series of experiments were carried out in vitro in order to determine the likely relative suitability of four major commercial lactase products used in this regard. The lactases displayed between 55 and 61% of maximum activity at 37 degrees C and significant activity between pH 3.0 and 6.5. They retained between 0 and 65% of original activities after exposure to full simulated digestive tract conditions for 6 h. All four enzymes proved to be particularly acid sensitive and only two products were enteric coated. The products demonstrated varying ability to hydrolyze lactose under simulated digestive tract conditions. The most effective product hydrolyzed 2.7 g lactose per capsule, suggesting that consumption of several capsules, as opposed to the usually recommended one or two, would be required to hydrolyze the entire 12 g lactose load characteristic of a dairy-based meal. All enzymes were substantially pure and displayed similar kinetic properties and molecular weights. None appeared ideally suited for use in the alleviation of lactose intolerance. The findings may in part explain the variability and often disappointing results previously reported for lactase-based clinical trials and will provide comparative baseline data against which candidate second-generation lactases may be assessed.