Abstract. For a quasilinear operator on the semiaxis a reduction theorem is proved on the cones of monotone functions in L p − L q setting for 0 < q < ∞, 1 ≤ p < ∞. The case 0 < p < 1 is also studied for operators with additional properties. In particular, we obtain critera for three-weight inequalities for the Hardy-type operators with Oinarov' kernel on monotone functions in the case 0 < q < p ≤ 1.