  We present a suite of binary evolution models with massive primaries (10
$\leq$ M$_1$ $\leq$ 40 M$_\odot$) and periods and mass ratios chosen such that
the systems undergo non-conservative mass transfer while the primaries have
helium cores. We track the total mass and chemical composition of the ejecta
from these systems. This material shows the abundance signatures of hot
hydrogen burning which are needed to explain the abundance patterns seen in
multiple populations in massive star clusters. We then calculate the total
yield of a population of binary stars with masses, mass ratios, and periods
consistent with their distribution in a field population. We show that the
overall abundance of this material is enriched in helium, nitrogen, sodium, and
aluminum, and depleted in carbon, oxygen, and magnesium, by amounts that are
consistent with observations. We also show that such a population of binaries
will return approximately 25% of its mass in this ejecta (compared to 4% if all
the stars were single), over a characteristic timescale of about 12 Myr. We
argue that massive binaries must be seriously considered as a contributor to
the source of enriched material needed to explain the multiple populations in
massive clusters, since essentially all massive stars are formed in binaries or
higher order multiples, massive binaries are primarily formed in clusters, and
massive binaries naturally produce material of the right composition.
