A recently developed crossover equation of state (EOS) incorporates contributions from long-wavelength density fluctuations by renormalization-group theory. This EOS can satisfactorily describe the thermodynamic properties of chain fluids both far-from and near-to the critical region; it is used here to calculate the critical locus of a mixture. Because the calculations require much computation time, especially for ternary land higher) mixtures, an interpolation method is used as suggested by Redlich over 30 years ago. For a binary mixture, along the critical line that gives the critical temperature or critical pressure as a function of composition, the limiting slopes at the critical points of the pure components are explicitly derived from the criteria for a critical point. Logarithmic-hyperbolic interpolation equations are selected to calculate the entire critical lines of the binary mixtures; this procedure is then generalized to multicomponent mixtures. Upon comparison with experimental critical data, the interpolation equations give good critical lines for binary and multicomponent type-1 mixtures of n-alkanes.