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The separation of lactic acid from lactose in the ultrafiltration permeate of cheese whey broth was studied using a cross-flow nanofiltration membrane unit. Experiments to test lactic acid recovery were conducted at 3 levels of transmembrane pressure (1.4, 2.1 and 2.8 MPa), two levels of initial lactic acid concentration (1.7% and 2.7%), and two types of nanofiltration membranes (DS- 5DK and DS-5HL). Higher transmembrane pressure caused significantly higher permeate flux and higher lactose and lactic acid retention ratio (P<0.0001). Higher initial lactic acid concentrations also caused significantly higher permeate flux, but significantly lower lactose and lactic acid retention (P<0.0001). The two tested membranes demonstrated significant differences on the permeate flux and lactose and lactic acid retention. Membrane DS-5DK was found to retain 100% of lactose at an initial lactic acid concentration of 18.6 g/L for all the tested pressures, and had a retention level of 99.5% of lactose at initial lactic acid concentration of 27.0 /L when the transmembrane pressure reached 2.8 MPa. For all the tests when lactose retention reached 99-100%, as much as 64% of the lactic acid in the permeate could be recovered.