  The completion $\overline{A}[\tau]$ of a locally convex *-algebra $A [ \tau
]$ with not jointly continuous multiplication is a *-vector space with partial
multiplication $xy$ defined only for $x$ or $y \in A_{0}$, and it is called a
topological quasi *-algebra. In this paper two classes of topological quasi
*-algebras called strict CQ$^*$-algebras and HCQ$^*$-algebras are studied.
Roughly speaking, a strict CQ$^*$-algebra (resp. HCQ$^*$-algebra) is a Banach
(resp. Hilbert) quasi *-algebra containing a C$^*$-algebra endowed with another
involution $\sharp$ and C$^*$-norm $\| \|_{\sharp}$. HCQ$^*$-algebras are
closely related to left Hilbert algebras. We shall show that a Hilbert space is
a HCQ$^*$-algebra if and only if it contains a left Hilbert algebra with unit
as a dense subspace. Further, we shall give a necessary and sufficient
condition under which a strict CQ$^*$-algebra is embedded in a HCQ$^*$-algebra.
