Record values of colossal magnetoresistance (CMR) have been achieved in the antiferromagnetic phase of the La1−xCaxMnO3system. At 125 K, the CMR of the La0.5Ca0.5MnO3reaches nearly 1 000 000%. It increases exponentially to 100 000 000% at 57 K. While the ground state is primarily an antiferromagnet, application of a magnetic field induces a ferromagnetic alignment of spins that is highly beneficial to the electron conduction. Other ferromagnetic samples exhibit very sharp magnetic phase transitions, with which the magnetotransport is closely correlated. ©1995 American Institute of Physics.