This is a comprehensively revised version of the second named
author's preprint "On rational injectivity for pretheories. Relative case."
(no. 501 on this server). We prove a generic injectivity result for etale
cohomology which can be formulated as follows.
Theorem: Let W be a connected smooth semi-local scheme over a field k,
X --> W a smooth and proper morphism, n an integer prime to char(k) and
K a complex of etale sheaves of Z/nZ-modules on X such that the
cohomology sheaves are locally constant constructible and bounded below.
If w denotes the generic point of W, then for all integers q the
natural restriction map of etale hypercohomology groups
H^q_et(X, K) --> H^q_et (X_w, K)
is a (universal) monomorphism.
The result applies to any "extensible pretheory". The proof extends the
techniques of section 4 of V. Voevodsky's paper "Cohomological theory of
presheaves with transfers" to the relative case.