A systematic study of the doping of the Mn-sites by cobalt in three series of manganites — La 0.76Ba 0.24(Mn 1- xCo x)O 3 single crystals, La 2/3Ba 1/3(Mn 1- xCo x)O 3 and La(Mn 1- xCo x)O 3 ceramics has been performed. It was found that La(Mn 1- xCo x)O 3 annealed at 800°C in the range 0.4⩽ x⩽0.9 is a mixture of ferromagnetic domains with ordered Mn and Co ions and ionically disordered spin-glass domains. In the quenched samples the fraction of spin-glass-type component increases strongly. The La 2/3Ba 1/3(Mn 1- xCo x)O 3 solid solutions exhibit also an evidence for phase separation in the range 0.5⩽ x⩽0.8. All the La(Mn 1- xCo x)O 3 samples show an insulating behavior, however, magnetoresistance reduces strongly when the cobalt content rises to x=0.5. The La 0.76Ba 0.24(Mn 1- xCo x)O 3 single crystals show first-order phase transition below their Curie points associated with a change of ground state of the Co 2+ ions. The magnetic phase diagrams are depicted. The results are discussed in terms of positive Mn 3+-O-Mn 4+, Mn 3+-O-Mn 3+, Mn 4+-O-Co 2+ and negative Mn 4+-O-Mn 4+, Co 2+-O-Co 2+, Co 2+-O-Mn 3+ superexchange interactions as well as Co 2+ and Mn 4+ ionic ordering.