Abstract We have investigated, theoretically and experimentally, the penetration of magnetic fields into ceramic HTSC for fields below the lower critical field of the grains. In the framework of a critical state theory in the low-field electrodynamics of a random Josephson medium, expressions for the harmonics of the induction B̄( t) as a function of external parameters were derived for two asymptotic cases — the case of the weak field and the case of the thin sample. The behavior of the magnetic induction B̄ was investigated in AC and DC fields in a wide range of temperature and field amplitudes. Rather good agreement between experimental and theoretical results has been found. From the experimental results the values of number of quantities characterizing the low-field electrodynamics of HTSC and their temperature dependences as well as an explicit expression for the critical current density j c(H, T) = j 0(T)H 2 0(T) H 2+H 2 0(T)) were derived.