We have calculated a correction of order m alpha(7) In-2 alpha to energy le
vels of the general two-body system of spin-1/2 particles with arbitary mas
ses. The result allows for the improved theoretical predictions of the 1S-2
S and 2S-2P intervals in positronium. Further implications to the hydrogen-
deuterium 1S-2S isotope shift are discussed also. [S1050-2937(99)00610-1].