Magnetotransport and magnetic properties of the system La 0 . 6 M 0 . 0 7 Ca 0 . 3 3 MnO 3 (M = Er, Yb, Bi) have been investigated. The behavior of the three samples varies much more than might be expected on the basis of the similarity in ionic radii of M 3 + ions and a small quantity of the substitution. A strong correlation is found between magnetization data, FMR and transport behavior. These results are discussed in terms of minor variations in lattice properties and differences in chemical properties, such as ionic radii and electronegativity, which influence the exchange mechanisms. The Bi containing compound is particularly interesting from an applications stand point, as it shows a sharp transition at around 265 K.