The low-field intragranular irreversibility line of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ high-temperature superconductors was obtained from transport critical-current measurements performed on polycrystalline bulk sintered samples with a pronounced granular character. For such samples, the intra- and intergranular critical-current densities are very different, the latter being largely mediated by tunneling. The method is based on the high sensitivity of the intergranular critical current to the local magnetic field at the intergrain contacts. The value of this field is strongly influenced by the grain magnetization and therefore depends on cooling conditions. The irreversibility temperature was determined as the temperature above which the voltage-current characteristics measured with the sample cooled in applied-field and in zero-field-cooling conditions become identical. For the investigated samples, an exponential decrease of the irreversibility field with increasing temperature was observed. A description of the irreversibility line in terms of thermally assisted flux flow is given.

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