We present results of a detailed analysis of ISEE 3 observations of low-energy protons (35--1600 keV) and low-frequency waves (period greater than 6 s) associated with the passage of the large oblique interplanetary shock of April 5, 1979. We observe a foreshock region for several hours prior to the arrival of the shock. Within this region we find (1) a peak in the proton energy spectrum at about 200 keV, (2) the proton energy density (in the range 35--1600 keV) comparable with the total energy density of the magnetic field, (3) a good correlation between the amplitude of the low-frequency waves and the 200-keV proton intensity, and (4) a good agreement between the peak in the wave power spectrum and the resonant frequency corresponding to the peak in the proton spectrum, only if large pitch angles are considered, suggesting that the protons are responsible for the waves by virtue of a resonant interaction. Our observations suggest that the high energy density of the high-energy solar flare protons preceding the shock could be responsible for ''seed'' waves which provide the scattering centers necessary for the acceleration of the lower-energy protons via a first-order Fermi acceleration mechanism.