A Peirce inner ideal J in an anisotropic Jordan * -triple A gives rise to a Peirce grading (J0, J1, J2) of A by defining J0 = J ⊥ , J1 = Ker(J ) ∩ Ker(J ⊥ ), J2 = J, where J ⊥ is the set of elements a of A for which {J a A} is equal to {0} and Ker(J ) is the set of elements a of A for which {J a J} is equal to {0}. It is shown that conversely, when A is a JBW * -triple factor, for each Peirce grading (J0, J1, J2) of A such that both J0 and J2 are non-zero, both J0 and J2 are Peirce inner ideals the corresponding Peirce decompositions of A being given by (J0)0 = J2, (J0)1 = J1, (J0)2 = J0; (J2)0 = J0, (J2)1 = J1, (J2)2 = J2. Mathematics Subject Classification (2000). Primary 17C65; Secondary 46L70.