  We study diffusive electron-electron interaction correction to conductivity
by analyzing simultaneously $\rho_{xx}$ and $\rho_{xy}$ for disordered 2D
electron systems in Si in tilted magnetic field. Tilting the field is shown to
be a straightforward tool to disentangle spin and orbital effects. In
particular, by changing the tilt angle we prove experimentally that in the
field range $g\mu_BB>k_BT$ the correction depends on modulus of magnetic field
rather than on its direction, which is expected for a system with isotropic
$g$-factor. In the high-field limit the correction behaves as $\ln (B)$, as
expected theoretically (Lee, Ramakrishnan, Phys. Rev. B{\bf 26}, 4009 (1982)).
Our data prove that the diffusive electron-electron interaction correction to
conductivity is not solely responsible for the huge and temperature dependent
magnetoresistance in parallel field, typically observed in Si-MOSFETs.
