The attachment characteristics of the Vero cell line on microcarriers has been found to be influenced by the serum proteins. Significant quantities of serum proteins were found to be adsorbed on the carrier surface, which resulted in decreased exchange capacity and a lower attachment rate constant. The kinetics of attachment followed primarily first order with respect to the concentration of free cells. With positively charged microcarriers the attachment process exhibited two distinct phases, an initial phase of normal attachment followed by a second phase characterized by a lower rate of attachment. The attachment rate constant in the first phase (0·062 min−1) was reduced to 0·01 min−1 in the second phase. Presumably, at a low concentration of free Vero cells, the attachment process is retarded by unadsorbed Bovine Serum Albumin (BSA) and other serum proteins present in the medium.