- A subspace Y of a separable metrizable space X is separable, but without X metrizable this is not true even If Y is a closed linear subspace of a topological vector space X. K.H. Hofmann and S.A. Morris introduced the class of pro-Lie groups which consists of projective limits of finite-dimensional Lie groups and proved that it contains all compact groups, locally compact abelian groups and connected locally compact groups and is closed under products and closed subgroups. A topological group G is almost connected if the quotient group of G by the connected component of its identity is compact. We prove that an almost connected pro-Lie group is separable iff its weight is not greater than c. It is deduced that an almost connected pro-Lie group is separable if and only if it is a subspace of a separable Hausdorff space. It is proved that a locally compact (even feathered) topological group G which is a subgroup of a separable Hausdorff topological group is separable, but the conclusion is false if it is assumed only that G is homeomorphic to a subspace of a separable Tychonoff space. Every precompact topological group of weight less than or equal to c is topologically isomorphic to a closed subgroup of a separable pseudocompact group of weight c. This implies that there is a wealth of closed nonseparable subgroups of separable pseudocompact groups. An example is presented under CH of a separable countably compact abelian group which contains a non-separable closed subgroup. It is proved that the following conditions are equivalent for an omega-narrow topological group G: (i) G is a subspace of a separable regular space; (ii) G is a subgroup of a separable topological group; (iii) G is a closed subgroup of a separable pathconnected locally pathconnected group.