  We prove that the flag kernel singular integral operators of
Nagel-Ricci-Stein on a homogeneous group are bounded on the Lp spaces. The
gradation associated with the kernels is the natural gradation of the
underlying Lie algebra. Our main tools are the Littlewood-Paley theory and a
symbolic calculus combined in the spirit of Duoandikoetxea and Rubio de
Francia.
