The recent development of methods for generating libraries of solid-state compounds has made it possible to apply combinatorial approaches to the discovery of materials. A library of 128 members containing different compositions and stoichiometries of Ln(x)M(y)CoO(delta), where Ln = Y or La and M = Pb, Ca, Sr, or Ba, was synthesized by a combination of thin-film deposition and physical masking techniques. Large magnetoresistance has been found in La-x(Ba,Sr,Ca)(y)-CoOdelta samples, whereas Y-based samples exhibit much smaller magnetoresistive effects. The magnetoresistance of the Co-containing compounds increases as the size of the alkaline earth ion increases, in sharp contrast to Mn-containing compounds, in which the magnetoresistance effect increases as the size of the alkaline earth ion decreases.