[en] The nature of the training effect in exchange bias compounds, which consists in a decrease in the exchange bias field H_EB under magnetic field cycling, is clarified. It is shown that the training effect in GdBaCo₂O_{5 + δ} cobaltite with δ ≈ 0.5 is caused by a decrease in magnetization. Depending on time, magnetization M(t) decreases sharply after the first cycle and slowly in subsequent N cycles. A direct relationship (proportionality) is established between the decrease in the magnetization M(t) of ferromagnetic particles and the change of the field H_EB(t). It is assumed that the cooling of a sample in a magnetic field gives rise to small (single-domain) and large ferromagnetic clusters in an antiferromagnetic environment. The sharp decrease in M(t) and H_EB(t) and the deviation of the function H_EB(N) from the well-known power-law relation ΔH_EB ∝ N^–1/2 with N > 1 is explained by the transformation of large ferromagnetic clusters into a multidomain state under magnetic field cycling. The results are consistent with the phenomenological model of Meiklejohn and Bean, which predicts a linear dependence of the exchange bias on the ferromagnetic magnetization of a ferromagnet/antiferromagnet structure.