Baby hamster kidney (BHK) cells were grown in stirred spinner flasks at 45 rpm, using both serum-containing and serum-free media. In both studies natural aggregates were formed. Aggregate formation is independent of cell growth or medium composition, but aggregate final properties are dependent on cell growth. Serum concentrations were varied between 0 and 20% without the addition of any serum substitutes. Aggregate characteristics (diameter, fraction of cells in aggregates, and compactness) remained constant above 5% serum, clearly indicating that there is no direct dependence of aggregate properties on serum above that concentration. Cell growth decreased with serum concentration below 5%. A serum-free medium with similar basal medium and supplemented with several additives was tested. Cell growth, cell viability, and aggregate characteristics remained unchanged when serum-free medium was compared with serum-containing medium with more than 5% serum at the same agitation rate. Using the serum-free medium, agitation rare was varied between 25 and 60 rpm. No differences in cell growth or viability were observed, but aggregate diameter was largely reduced with the increase in agitation rate (from 200 to 90 mu m); this clearly indicates that the control of the size of aggregates by vessel hydrodynamics previously reported for serum-containing medium is independent of the medium composition including absence or presence of serum, for the two tested media.