Abstract. Given a graded complete intersection ideal J = (f 1 , . . . , fc) ⊆ k[x 0 , . . . , xn] = S, where k is a field of characteristic p > 0 such that [k : k p ] < ∞, we show that if S/J has an isolated non-F-pure point then the Frobenius action on top local cohomology H n+1−c m (S/J) is injective in sufficiently negative degrees, and we compute the least degree of any kernel element. If S/J has an isolated singularity, we are also able to give an effective bound on p ensuring the Frobenius action on H n+1−c m (S/J) is injective in all negative degrees, extending a result of Bhatt and Singh in the hypersurface case.