Cryopreserved human hepatocytes in suspension were used to estimate in vivo hepatic clearances for six different drugs. In vitro intrinsic clearances were measured on the basis of substrate depletion. The number of different hepatocyte donors required for a reasonable estimate of in vivo hepatic clearance--within twice or (1/2) of the actual value--was determined. Depending upon the desired level of confidence, anywhere from 9-20 donors are required by this method.