Static-susceptibility and thermal-expansion measurements indicate that Ce3Bi4Pt3 is mixed-valent-like, whereas transport measurements reveal a gap of Δ=5 meV in the electronic excitation spectrum. We report the magnetic response of Ce3Bi4Pt3, at temperatures between 2 and 150 K, determined with incident neutron energies of 3.1, 17, and 69 meV. The isostructural, nonmagnetic reference compound La3Bi4Pt3 was also studied at the same temperatures and energies so that phonon contributions could be subtracted accurately. At 2 K, a gap Δm=12 meV is present in the magnetic excitation spectrum: the 3.1-, 17-, and 69-meV data consistently show that the magnetic intensity is zero for energy transfers less than 12 meV, rises sharply between 12 and 20 meV, and falls off smoothly beyond. Above 50 K, the gap in the spin-spin correlation function starts to fill. The neutron results reproduce the temperature dependence of the bulk susceptibility. The relationship between these and other measurements will be discussed. © 1991 The American Physical Society.