EN
La_{1-x} Ca_{x}MnO_{3} perovskites show semiconducting properties in the paramagnetic range. The gap is reduced near x=0.33, where ferromagnetic ordering is observed. The ferromagnetic ordering then induces a semiconductor-metal transition, and gives rise to a giant magnetoresistance effect. The ground state electronic structure calculations were done with KKR-CPA method for hypothetical cubic and ferromagnetic LaMnO_{3} and CaMnO_{3} compounds, as well as for disordered La_{1-x}Ca_{x}MnO_{3} (x=0.33, 0.4, 0.5) alloys with real crystal data. As a result, we get a magnetic moment per formula 4.00μ_{B} and 3.00μ_{B} and half-metallic behaviour for end-compounds, respectively. In the ferromagnetic region a linear decrease in the magnetic moment of La_{1-x}Ca_{x}MnO_{3} is observed, together with the decrease in the gap width for spin-down carriers, if doping Ca in La_{1-x}Ca_{x}MnO_{3}. A simple model is developed, which describes magnetic and transport properties as resulting from an exchange-induced band-crossing semiconductor-metal transition, as for instance in EuO.