Using the Mariner 5 solar wind plasma and magnetic field data, we present observations of field-aligned suprathermal proton distribution having pronounced high-energy shoulders. These observations, similar to the interpenetrating stream observations of Feldman et al.(1974), are clear evidence that such proton distributions are interplanetary rather than bow shock associated phenomena. Large speed is found to be a requirement for the occurrence of suprathermal proton distributions; further, we find the proportion of particles in the shoulder to be limited by the magnitude of the speed. It is suggested that this last result could indicate that the proton thermal anisotropy is limited at times by wave-particle interactions.