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The results of magnetic susceptibility, magnetization, electrical resistivity and specific heat measurements performed on Ho 3 Co single crystals show that this compound exhibits two different antiferromagnetic structures: AF II at 8 K < T < 22 K and AF I below T t ≈ 8 K. Below the Néel temperature T N = 22 K the application of a magnetic field along the main crystallographic directions induces magnetic phase transitions which are accompanied by giant magnetoresistance. At T < T t the field-induced phase transitions along the c-and b-axes are found to be irreversible, and a small ferromagnetic component is observed along the a-axis. These peculiarities are associated with the non-Kramers character of the Ho ion and with the presence of a complex incommensurate magnetic structure of Ho 3 Co below T N . The temperature coefficient of the electrical resistivity for Ho 3 Co above T N over a wide temperature range is found to differ from that observed for other R 3 Co compounds. Such a behaviour is attributed to the presence of an additional contribution to the conduction electron scattering by spin fluctuations induced by f-d exchange in the itinerant d-electron subsystem. The value of this extra contribution and its temperature range is suggested to depend on the spin value of the R ion. The excess of the effective magnetic moment per R ion, which is observed in Ho 3 Co and in other R 3 M type compounds, is also attributed to spin fluctuations induced by f-d exchange.