We study the singular integral operator
$$
T_{\Omega,\alpha}f(x) = \pv \int_{R^n} b(|y|) \Omega(y')
|y|^{-n-\alpha} f(x-y)\,dy,
$$
defined on all test functions $f$,where $b$ is a bounded function, $\alpha\geq 0$,
$\Omega(y')$ is an integrable function on the unit sphere $S^{n-1}$ satisfying
certain cancellation conditions. We prove that, for $1