Added Nα-p-tosyl-l-arginine methyl ester or Nα-benzoyl-l-arginine ethyl ester inhibited the stimulation by insulin of phosphorylation of the 95,000 dalton subunit of the insulin receptor both in a partially purified insulin receptor fraction from rat adipocytes and in a highly purified insulin receptor preparation from human placenta. N-α-p-tosyl-l-lysine chloromethyl ketone, Nα-p-tosyl-l-lysine methyl ester, or N-acetyl-l-phenylalanine ethyl ester were much less potent, while N-benzoyl-l-alanine methyl ester was without effect. Inhibition of the phosphorylation by the arginine analogues did not require preincubation of the insulin receptor with inhibitors in the presence of insulin prior to phosphorylation. Inhibition by Nα-p-tosyl-l-arginine methyl ester was decreased by preincubation of the receptor fraction with cold ATP and MnCl 2. These results suggest that Nα-p-tosyl-1-arginine methyl ester inhibits an initial ATP and Mn 2+ dependent reaction in insulin-stimulated phosphorylation process.