Antagonistic effects of pharmacologically diverse agents on the growth-inhibitory action of IFN-α2a. Sensitive (D_W^S) Daudi cells were continuously exposed to serial twofold dilutions of IFN-α2a for 48 hours. Growth inhibition was measured by MTT assay. Ribbon graphs show antagonism when STSP (125 mmol/L), PMA (10 nmol/L), or ATA (125 μmol/L) were added to the IFN-α from 18 to 48 hours. Neither the PMA nor ATA themselves inhibited cell growth during this 30-hour period. STSP alone inhibited cell growth by 28%, but was nevertheless effective in decreasing the inhibitory action of IFN-α.