METABOLIC cooperation is a form of intercellular communication by which cells in contact exchange molecules, a process providing multicellular organisms with an important mechanism for control of metabolic activity1. Subak-Sharpe et al.2 observed contact-dependent transfer of purine nucleotides from normal cells to cells mutationally incapable of producing inosinic acid due to deficiency in hypoxanthine-guanine phosphoribosyl- transferase3. Metabolic cooperation of this type was later also demonstrated with other enzymic markers, such as adenine phosphoribosyltransferase and thymidine kinase4,5. Such transfers are characterised, by the normal cell being the donor and the mutant cell being the recipient, the former transferring to the latter a mutationally lacking metabolite. We report here on a new form of contact-dependent metabolic cooperation, unique in that the transfer of a metabolite occurs from a mutant donor cell to a normal recipient cell.