Assn,ACT. It is proven that the group ring of an amalgamated free product of residually torsion free nilpotent groups is a domain and can be embedded in a skew field. This is a generalization of J. Lewin's theorem, proven for the case of free groups. Our proof is based on the study of the Malcev-Neumann power series ring K < G) of a residually torsion free nilpotent group G. It is shown that its subfield D, generated by the group ring KG, does not depend on the order of G for many kinds of

more »

... s and the study of D can be reduced in some sense to the case when G is nilpotent.