The relative yield of π+ mesons has been determined for 335-Mev protons on several elements. Specifically, the relative differential cross section, d2σdEdΩ, was measured at 0°+/-4° in the reaction p+A-->π++A' for D, Be9, B10, B (natural), C, Al, Cu, Fe, Ag, and Pb. The π+-meson energies ranged from 34 to 147 Mev. They were counted electronically from signals generated in a trans-stilbene crystal telescope. Identification of the π+ meson depended upon a coincidence in the first two crystals plus the π+-μ+ decay in the third crystal of the telescope. No special isotope effects were observed for the π+-meson spectra from the isotopes Be9, B10, B (natural), and C. For C, Cu, and Pb the relative π+-meson yield per nucleus is proportional to the product of two factors, (a) the number of protons in the nucleus, (b) an attenuation factor F which is derived from proton and meson mean free paths in nuclear matter. These mean free paths are consistent with those calculated from measured interaction cross sections of protons and mesons with C, Cu, and Pb. The importance of meson scattering within nuclear matter is discussed in the case of carbon. Also the total π+-meson production is estimated for carbon from the experimental proton-proton cross section and is compared with the experimental results for carbon.