Comparisons of antagonistic potency in cells cultured in 10% serum versus whole blood allow us to determine whether the high concentration of proteins/lipoproteins present in serum inhibit E5564 activity. In all systems but the rat, antagonistic activity of E5564 in cultured cells was within fourfold that measured in high serum (blood) compared with assays done in low-serum conditions (cultured cells or monocytes) (60, 63) . This indicates that serum has little or no inhibitory effect on antagonistic activity under these in vitro conditions. However, extended incubations in whole blood demonstrated that activity of E5564 was measurably reduced.