Let X be a Banach space, $A:D(A){subset}X{ ightarrow}X$ the generator of a compact $C_0-semigroup;S(t):X{ ightarrow}X,;t{geq}0$, D a locally closed subset in X, and $f:(a,b){ imes}C([-q,0];X){ ightarrow}X$ a function of Caratheodory type. The main result of this paper is that a necessary and sufficient condition in order that D be a viable domain of the semi linear differential equation of retarded type $$u#(t)=Au(t)+f(t,u_t),;t{in}[t_0,;t_0+T],{u_t}_0={phi}{in}C([-q,0];X)$$ is the tangency condition $$limits_{h{downarrow}0}^{lim;inf;h^{-1}d(S(h)v(0)+hf(t,v);D)=0}$$ for almost every $t{in}(a,b)$ and every $v{in}C([-q,0];X);with;v(0){in}D$.