The effect of exchange of 15-hydroxyl for fluor on biological activity of PGF2 alpha and PGE2 on isolated smooth muscles of different organs has been studied. The exchange leads to significant increase in contractile (100-fold) and relaxation (1000-fold) activity of PGF2 alpha on smooth respiratory muscles. At the same time, the effect of 15-fluor-15-deoxyprostaglandin F2 alpha on smooth muscles of intestinal and vascular tracts did not differ from that of PGF2 alpha. Similar modification of PGE2 led to the decrease (10-fold) both in contractile and relaxation activity on all studied types of smooth muscles. The data obtained have been discussed within the boundaries of prostanoid receptor classification (Kennedy, 1982). Fluor derivative of PGF2 alpha may be used for pharmacological differentiation of EP-receptors.