Eukaryotic initiation factor (eIF) 4GI is efficiently cleaved during picornaviral replication. eIF4GI processing has also recently been observed during HIV-1 replication. We have compared the efficiency of eIF4GI proteolysis in rabbit reticulocyte lysates during translation of mRNAs encoding the foot-and-mouth disease virus leader proteinase (L(pro)) or the HIV-1 proteinase (HIV-1(pro)). L(pro) cleaved 50% eIF4GI within 12 min whereas HIV-1(pro) required 4 h; the concentrations were 2 pg/microl (0.1 nM) for L(pro) and 60 pg/microl (2.66 nM) for HIV-1(pro). HIV-1(pro) processing of eIF4GI is therefore not quantitatively analogous to that of L(pro), suggesting that the primary function of eIF4GI cleavage in HIV-1 replication may not be protein synthesis inhibition.