The cold promoted activation of factor VII was induced by treatment with oestrogenic hormones, which increased the frequency of cold activation positive plasmas from under 20 to about 65%. The spontaneous occurrence in untreated subjects was probably also correlated with the oestrogen level. In pregnancy, the frequency of cold activation positive plasmas was about 93%. The participation of hormones other than oestrogens was not indicated. Cold activation was obtained with both steroids and diethylstilboestrol. Neither of these had any in vitro effect. It is suggested that the oestrogenic effect is mediated by an activator.