  4-dimensional spaces equipped with 2-dimensional (complex holomorphic or real
smooth) completely integrable distributions are considered. The integral
manifolds of such distributions are totally null and totally geodesics
2-dimensional surfaces which are called the null strings. Properties of
congruences (foliations) of such 2-surfaces are studied. Some relations between
properties of congruences of null strings, Petrov-Penrose type of SD Weyl
spinor and algebraic types of the traceless Ricci tensor are analyzed.
