BACKGROUND: Alpha hydroxy acids such as lactic and glycolic acids (AHAs) accelerate epidermal turnover and exfoliate the stratum corneum. The roles of pH and concentration in these antiaging effects of AHAs is unclear, but a lower (more acidic) pH and higher concentration of acid are thought to be more effective. OBJECTIVE: To examine the effects on skin renewal rates of lactic acid 10% at pHs of 2.0, 3.0, and 4.0, and of 5%, 10% and 15% lactic acid at a pH of 3.0. METHODS: Twenty-six female subjects participated in the double-blind, randomized, placebo-controlled study. The dansyl chloride stain technique was used to measure epidermal renewal times. RESULTS: Both pH and concentration are critical in the lactic acid effect. At a fixed lactic acid concentration, the desquamative effect was highly pH dependent. At a fixed pH, the turnover rate of skin was concentration dependent. CONCLUSION: The desquamative and proliferation-stimulating effects of lactic acid are very pH and concentration dependent, suggesting the "free acid" concentration is the active moiety.