Let R(m,n) be the ring of highest weight vectors of the action of O-m
x GL(n) on the polynomial algebra of m x n matrices. We determine R(m,
2) and find generators for R(m,3). In particular, the results about R(
m,2) give branching rules and information about the structure of holom
orphic representations of Sp(4). (C) 1995 Academic Press, Inc.