The carboxyl reagent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) inactivated ATPase activities of isolated MF 1 and BF 1 when assayed in an MgCl 2 medium, but not in an EDTA medium. However, another carboxyl reagent, N,N'-dicyclohexylcarbodiimide (DCCD) was found to inhibit MF 1 and BF 1 when assayed either in the presence of MgCl 2 or EDTA. These data suggest that EEDQ interferes with the binding of Mg 2 + at catalytic sites of both MF 1 and BF 1 and that EEDQ on one hand, and DCCD on the other, react with different carboxyl groups on MF 1 and BF 1 .