The effect of calcium doping on the magnetic and transport properties of the intermediate size lanthanide cobaltites of the type Ln 1− x Ca x BaCo 2O 5.50± δ (Ln=Y, Gd, Eu and Sm) has been investigated for 0⩽ x⩽0.2. The substitution of Ln by calcium induces a large expansion of the ferromagnetic state in the whole temperature range below T C. The unusual trend of the decrease in T C with the increase in Ln size in the undoped parent oxides becomes opposite in the calcium doped samples. Such an unusual behavior of the ferromagnetic T C in the parent compounds is explained on the basis of thermally activated hole-mediated ferromagnetic coupling between the high-spin cobalt ions. The ferromagnetic state of the Ca-doped samples originates from the Co 3+–O–Co 4+ superexchange interaction, where Co 4+ emerges from the disproportionation mechanism of the cobalt Co 3+ into Co 2+ and Co 4+. However, the Ca-doping does not significantly affect the metal–insulator transition, which is associated with the structural change and not related to the spin state transition.