Several different combinations of serum supplements and also serum-free media were examined for their ability to support the growth of MRC-5 and Vero cells on Cytodex microcarriers. Greater economy of serum was achieved for routine microcarrier culture by selecting the type of serum supplement, on the basis of whether the supplement was to support attachment and growth of cells at low densities or growth of cells at later stages in the culture cycle. Further economy was achieved by altering the serum concentration according to the requirements of each stage of the culture cycle. More consistent results were obtained when batches of serum were selected on the basis of quality control tests with microcarrier culture as well as with monolayer culture. Low serum and serum-free media were able to support the growth of cells in microcarrier cultures. A mixture of DME/Medium 199 (50/50) containing foetal calf serum (0.5%, v/v), BSA and EGF supported growth of MRC-5 and Vero cells to nearly the same extent as DME/Medium 199 containing 10% (v/v) foetal calf serum. Serum-free media was used to achieve cell yields at least one half of those attained with serum-supplemented media. Maximum growth was obtained when DME/Medium 199 was supplemented with fibronectin, BSA, insulin, transferrin, putrescine, fetuin and EFG. Fetuin was omitted and replaced by dexamethasone and trace metals with only a slight reduction in cell yield. Fibronectin was essential in all serum-free media.