Crystallographic and magnetic studies of the LaPt 5 - x M x and NdPt 5 - x M x (M Al, Ga, In) systems have been carried out. It was found that platinum in LaPt 5 and NdPt 5 can be replaced by aluminium or gallium preferentially in the intermediate layer of the prototype structures up to LaPt 2 Al 3 , LaPt 2 Ga 3 , NdPt 3 Al 2 and NdPt 3 Ga 2 respectively. However, substitution of indium for platinum was possible only in the system NdPt 5 - x In x and at the composition NdPt 4 In, but with a change to the cubic crystal structure of the MgSnCu 4 type. Magnetic measurements indicate that the Curie-Weiss behaviour of NdPt 5 is retained in all single-phase ternary samples. Generally the negative paramagnetic Curie temperatures decrease and the effective magnetic moments increase as the content of the substituent increases. In the lanthanum-containing systems the diamagnetism of LaPt 5 changes to a weak paramagnetism when platinum is replaced by aluminium or gallium.