  We compute long-distance interaction potentials between certain 1/2 and 1/4
supersymmetric D-brane configurations of type IIB theory, demonstrating
detailed agreement between classical supergravity and one-loop instanton matrix
model results. This confirms the interpretation of D-branes as described by
classical matrix model backgrounds as being `populated' by large number of
D-instantons, i.e. as corresponding to non-marginal bound states of branes of
lower dimensions. In the process, we establish precise relation between matrix
model expressions and non-abelian F^4 terms in the super Yang-Mills effective
action.
