The kinetic mechanism of the human platelet S-adenosyl-L-methionine (AdoMet)-linked isoprenylated protein methyltransferase was studied and determined to be ordered bibi. AdoMet binds first, and S-adenosyl-L-homocysteine (AdoHcy) departs last. Simple N-acetylated farnesylated cysteine analogs, such as N-acetyl-S-farnesyl-L-cysteine (AFC), are excellent substrates for the enzyme. Although many N-acetylated farnesylated cysteine analogs are excellent substrates for the enzyme, analogs with bulky moieties adjacent to the farnesylcysteine are neither substrates nor inhibitors of the enzyme. Two molecules of this class, N-benzoyl-S-farnesyl-L-cysteine (BzFC) and N-pivaloyl-S-farnesyl-L-cysteine (PFC) are useful in sorting out the putative physiological role of the methyltransferase in mediating human platelet aggregation because their pharmacological activities are unlinked to methyltransferase inhibition. When studied as inhibitors of platelet aggregation, the analogs are as active, or more active, than bona fide methyltransferase inhibitors of similar structure. Therefore, although it is possible that methyltransferase inhibitors, such as AFC, inhibit the enzyme when applied to cells, the observed pharmacological effects appear to be unrelated to this blockade. The new FC analogs described here have revealed a new signal transduction target which will be of some interest to explore.