Guinea pigs sensitized with either the trivalent chromium chloride or the hexavalent potassium dichromate are capable of reacting in vivo and in vitro to challenges with both chromium salts. This double reactivity is retained also after repeated restimulations with only 1 of these chromium compounds. From the failure to select lymphocytes directed specifically against a chromium determinant of a particular valence it is concluded that by sensitization with chromium salts of different valences a common determinant or closely related determinants are formed. It is suggested that this determinant is formed by chromium in the trivalent form.