On the Root-Class Residuallity of Generalized Free Products

Let $\mathcal{K}$ be a root class of groups. It is proved that a free product of any family of residually $\mathcal{K}$ groups with one amalgamated subgroup, which is a retract in all free factors, is residually $\mathcal{K}$. The sufficient condition for a generalized free product of two groups to be residually $\mathcal{K}$ is also obtained, provided that the amalgamated subgroup is normal in one of the free factors and is a retract in another.