  We apply the techniques developed by I. Panin for the proof of the
equicharacteristic case of the Serre-Grothendieck conjecture for isotropic
reductive groups (I. Panin, A. Stavrova, N. Vavilov, 2015; I. Panin, 2019) to
obtain similar injectivity and A^1-invariance theorems for non-stable
K_1-functors associated to isotropic reductive groups. Namely, let G be a
reductive group over a commutative ring R. We say that G has isotropic rank
>=n, if every normal semisimple reductive R-subgroup of G contains (G_m)^n. We
show that if G has isotropic rank >=2 and R is a regular domain containing a
field, then K_1^G(R[x])=K_1^G(R) for any n>=1, where K_1^G(R)=G(R)/E(R) is the
corresponding non-stable K_1-functor, also called the Whitehead group of G. If
R is, moreover, local, then we show that K_1^G(R)->K_1^G(K) is injective, where
K is the field of fractions of R.
