Compounds in the system L1-xPrxBa1.7Sr0.3Cu3O7 (L=Yb and Lu) were synthesized to study the effect of Pr substitution on superconductivity. The nature of variation of Tc with Pr concentration is significantly different from that in other Pr-doped L-123 systems and deviates from Abrikosov-Gor’kov-like behavior. The results are explained on the basis of hole localization and magnetic pair-breaking mechanisms. The ion size effect on Tc is attributed to the dependence of the extent of orthorhombicity and oxygen ordering on the L ion leading to retention of mobile holes in the CuO2 planes. © 1995 The American Physical Society.