The substitution of Tb for Tm in the Laves phase compound TmCo 2 leads to appearance of a magnetic moment on the Co atoms through the metamagnetic transition in the itinerant d-electron subsystem and gives rise to long-range ferrimagnetic order in Tm 1−x Tb x Co 2 at x ജ 0.15. The magnetic state of the compound Tm 0.9 Tb 0.1 Co 2 , i.e., just below the critical Tb concentration, is characterized by the presence of large regions with short-range magnetic order and localized spin fluctuations ͑LSFs͒ induced in the Co 3d-electron subsystem by the fluctuating f-d exchange due to the Tm-Tb substitution. The peculiar magnetic state of this compound is strongly influenced by an external magnetic field which produces a first-order magnetic phase transition to a long-range ferrimagnetic state with the magnetic moment on the Co atoms up to ͑0.7-0.8͒ B. This field-induced transition in Tm 0.9 Tb 0.1 Co 2 is found to be irreversible. It is accompanied by a giant and irreversible reduction of the electrical resistivity ͑⌬ / ϳ −45% ͒, specific heat ͑by about 3.7 times at 2 K͒, and intensity of magnetic neutron scattering. Such behavior is associated with the field-induced metamagnetic transition in the itinerant d-electron subsystem mediated by the f-d exchange. Significantly enhanced values of the residual resistivity and the coefficient ␥ of the T-linear contribution to the specific heat in the compound with x = 0.1 as well as their unusual behavior with temperature and under application of the magnetic field is ascribed to the presence of LSF.