The intergranular critical-current density Jc as a function of the local internal field Hi has been studied by AC susceptibility measurements for a sintered 123-YBCO superconductor at 77 K. Two samples cut from the same pellet are used for this purpose. Our results show that instead of Bean's, Kim's and the power-law models, the exponential-law and the triangular-pulse (linear) critical-state models are appropriate to fit field-dependent AC susceptibility data, so that they can be a good basis for Jc(Hi) determination from AC susceptibility measurements. The estimated Jc(Hi) curve has a linear low-field part, and the excellent exponential fit in a wide field range reflects a transition from the linear to a power-law-type Jc. This behavior is also found in low-field AC- and DC-magnetic measurements of many other sintered samples. Vortex pinning quantities obtained from Kim's model fit appear to be meaningless. © 1991.