The present invention relates to recombinant protein PEG positional isomers. More specifically, it relates to PEG positional isomers of an antibody having specificity for antigenic determinants of human tumor necrosis factor alpha (TNF.alpha.). More specifically, it relates to PEG positional isomers of CDP870. The present invention also relates to compositions comprising the PEG positional isomers and therapeutic uses of the composition for treating a disease medicated by TNF.alpha. including acute and chronic immune and immunoregulatory disorders and rheumatoid-or osteo-arthritis.