The expression of two discrete cell properties related to the host natural effector mechanisms, i.e., resistance to damage by H2O2, a cytotoxic product of activated macrophages, and the ability to secrete PGE, which inhibits NK-cell cytotoxicity, has been examined in parental Syrian hamster embryo cells spontaneously transformed in vitro (STHE strain) and in 18 in vivo selected sublines. In all cell variants, resistance to H2O2 and PGE-releasing activity were either both expressed, or not expressed at all. Parental STHE cells and 5 variants selected in vivo, which were equally highly susceptible to H2O2-induced damage, did not release any detectable amount of PGE upon contact with NK cells. In contrast, 13 other STHE variants selected in vivo and characterized by their resistance to H2O2, all released PGE upon contact with NK cells. Thus, these two biochemically unrelated cell phenotypic characteristics are likely to be either simultaneously selected in vivo, or united in cluster which pre-exist or appear in rare cell variants of the parental cell population in the conditions of in vivo natural selection pressure.