The growth of Madin Darby canine kidney cells on microcarriers was studied at different cell seeding densities. Maximum growth was attained when a cell-to-bead ratio of 7:1 was used. Under these conditions an initial concentration of above 3 X 10(6) cells/ml resulted in a yield of over 2 X 10(6) cell/ml in 2 d. The amino acid utilization of cells from Dulbecco's modified Eagle medium was studied under the above conditions. Eight amino acids (arg, cys, gln, ile, leu, met, ser, and val) showed rapid and near complete depletion from the medium. Five amino acids (his, lys, phe, thr, and tyr) showed limited depletion. Two amino acids (ala and gly) were released into the medium by the cells.