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-Q2 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 λ = 0.3 for masses between 150 and 280 GeV.