The multiplication of Clr/nr~~&rt~orfns cells can be arrcstcd by the spirolactonc derivative RU 26752 and this is li~lly rcversiblc by the natural hormone nldosteronc. Continuous growth in the prcscnce of RU 26752 led to the isolation of a population subsequently resistant to the action of mincralocortoid analogucs. due possibly to the sclcclion of mutant cells. lmmunopbotochcmical cvidemx is provided Ibr a 52 kDa protein that possesses functional steroid and DNA binding domains. Alga cells thcrcrorc appear to respond to steroid hormones in a manner similar to Lhe mammalian systems, possibly via a receptor that may represent a pygmy ancestor ol'thc latter day steroid receptor superfamily.