Cells of a genetic variant of the hamster fibroblast line BHK 21 which lack inosinic pyrophosphorylase activity ( IPP - cells) and therefore cannot normally incorporate [ 3 H]hypoxanthine were grown in mixed culture with cells of BHK 21 sublines which have inosinic pyrophosphorylase activity ( IPP + cells). If not in contact with IPP + cells, IPP - cells do not incorporate added [ 3 H]hypoxanthine into nucleic acid. IPP + cells always do incorporate [ 3 H]hypoxanthine and IPP - cells when in direct or indirect contact with IPP + cells also incorporate the isotope. Cell to cell contact appears to be essential for this gain of a metabolic function by IPP - cells. The possible molecular basis and general implications of the phenomenon are discussed.