  The e^+ jet invariant mass spectrum produced in the reaction e^+ p -> e^+ X
has been studied at a center-of-mass energy of 300 GeV. The data were collected
using the ZEUS detector operating at the HERA collider, and correspond to an
integrated luminosity of 47.7 pb^{-1}. The observed mass spectrum is in good
agreement with Standard Model expectations up to an e^+ jet mass of 210 GeV.
Above this mass, some excess is seen. The angular distribution of these events
is typical of high-Q^2 neutral current events and does not give convincing
evidence for the presence of a narrow scalar or vector state. Limits are
presented on the product of cross section and branching ratio for such a state
and are interpreted as limits on leptoquark or R-parity-violating squark
production. Specific leptoquark types are ruled out at 95% confidence level for
coupling strength lambda=0.3 for masses between 150 and 280 GeV.
