Interfacial tensions between polystyrene (PS) and poly(methyl methacrylate) (PMMA) were measured as a function of PS molecular weight at 190-degrees-C via imbedded-fiber retraction (IFR). The dynamic experiment enabled studies of much higher molecular weight resins than probed to date with equilibrium experiments, and this ability was applied to confirm the presence of the high-molecular-weight limiting interfacial-tension plateau for this system with direct measurements. Only narrow-polydispersity (M(w)/M(n) < 1.3) samples were studied. The number-average molecular weight (M(n)) for the set of 10 PS samples ranged from 2 to 200 kg/mol. The M(n) of the PMMA sample was 24.4 kg/mol. The interfacial tension (gamma12) followed a gamma12infinityM - C1M(n)-z dependence at 190-degrees-C. For M(n) exceeding about 48 kg/mol, gamma12 was independent of M(n) with a mean value (gamma12infinityM) of 1.2 +/- 0.1 dyn/cm. The scaling exponent (z), which summarizes the strong growth of gamma12 at lower M(n), was estimated to be 0.73 +/- 0.24. The results are compared to gamma12 data from the pendant-drop technique and to theoretical predictions.