We study the effects of bulk (Dresselhaus) and structural (Rashba) spin-orbit interaction terms on the low-field magnetoresistance in high-quality 2D systems. The theory accounting for both of these terms valid in the whole range of classically weak magnetic fields is proposed. The suppression of antilocalization correction as Rashba and Dresselhaus terms strengths approach each other is demonstrated. The effect of cubic in the wavevector spin-splitting term is analyzed.