Two series of derivatives of the complex CaCu3Mn4O12 perovskite have been prepared under moderate pressure conditions ( 20 kbar), and characterized by neutron powder diffraction, magnetic and magnetotransport measurements. In a susbtituted series with formula LnCu(3)Mn(4)O(12) (Ln = rare earths) we observe a significant increment of T-C and a level of low field magnetoresistance at RT that compete with the best values described for the state-of-the art perovskite oxide systems. (C) 2004 Elsevier B.V. All rights reserved.