  In this Letter we establish a relationship between symmetric SU(2)
Yang--Mills instantons and metrics with Spin(7)-holonomy. Our method is based
on a slight extension of that of Bryant and Salamon developed to construct
explicit manifolds with special holonomies in 1989.
  More precisely, we prove that making use of symmetric SU(2) Yang--Mills
instantons on Riemannian spin-manifolds, we can construct metrics on the chiral
spinor bundle whose holonomies are within Spin(7). Moreover if the resulting
space is connected, simply connected and complete, the holonomy coincides with
Spin(7).
  The basic example is the metric constructed on the chiral spinor bundle of
the round four-sphere by using a generic SU(2)-instanton of unit action; hence
it is a five-parameter deformation of the Bryant--Salamon example, also found
by Gibbons, Page and Pope.
