  Strongly self-dual Yang-Mills fields in even dimensional spaces are
characterised by a set of constraints on the eigenvalues of the Yang-Mills
fields $F_{\mu \nu}$. We derive a topological bound on ${\bf R}^8$, $\int_{M} (
F,F )^2 \geq k \int_{M} p_1^2$ where $p_1$ is the first Pontrjagin class of the
SO(n) Yang-Mills bundle and $k$ is a constant. Strongly self-dual Yang-Mills
fields realise the lower bound.
