The local spin density approximation (LSDA) is used to investigate the interrelationships between electronic properties, magnetism, structure, and transport in La1-xCaxMnO3. The LSDA is shown to provide a realistic description of the endpoint compounds, including magnetic and structural symmetry. Mn-O hybridization is strong and spin dependent. Supercell calculations for x = 1/4 and x = 1/3 yield ferromagnetic configurations. Strong local La-Ca cation effects are found, which lead to localization of the low density of minority spin carriers in this regime. Implications of this effectively half-metallic electronic structure are discussed.