Abstract: This paper deals with the bordism groups of manifolds with semi-free S1. actions. These groups are denoted by SF,(S1). We study the multiplicative structure by using a J-homomorphism map. We also study the construction K, which gives a set of multiplicative generators, and we present an algebraic interpretation of this geometric construction. Finally, as an application, we analyze the homomorphisms rp: SF.(S1) ? SF.(Z) from the bordism group of semi-free S1-actions on the bordism group of Z, actions induced by the restriction functors Ver menos