We present resistivity (T), susceptibility (T), and specific heat C(T) data for Ce3Bi4Pt3. The susceptibility exhibits a broad maximum centered near 80 K, typical of a somewhat-heavy-electron compound; were the material metallic, a linear coefficient of specific heat =75 mJ/mol Ce K2 would be expected. However, the compound is InotR metallic, as indicated by its resistivity which rises to large values at low temperatures and exhibits activated behavior with an activation energy /kB=35 K. By analogy to SmB6 and YbB12, this energy gap arises from 4f-electron conduction-electron hybridization. Due to the gap, electronic excitations are suppressed at low temperatures and the specific heat is smaller than in nonmagnetic La3Bi4Pt3. Alloying with lanthanum ((Ce1-xLax)3Bi4Pt3) decreases the resistivity and increases the specific heat towards the value expected for the metallic case; i.e., for moderate alloying (x=0.07) the behavior is that of a moderately disordered heavy-electron metal. We argue that lattice periodicity is an essential requirement for the formation of the hybridization gap. © 1990 The American Physical Society.