Hexadecylphosphocholine (HePC), an experimental and clinical antitumour agent of the alkyllysophospholipid group, was tested for its radiosensitising effect on a panel of nine human mammary cancer cell lines in vitro. Growth inhibition by ionising radiation and recovery from it were not influenced by pretreatment with HePC in most cases, except for two cell lines expressing an activated ras oncogene. In the latter we found an enhanced radioresistance that was abolished by pretreatment with HePC. Our results suggest that HePC may act as a radiosensitiser for cells carrying an activated ras oncogene.