An easy biochemical preparation of the prostaglandin endoperoxides, PGH1 and PGH2, is described. Both of the endoperoxides are potent contractors of isolated gerbil colon smooth muscle. Contracture with PGH2 is about equal to that caused by the standard, PGE1, while contracture with PGH1 is about half of that caused by PGE1. PGH1 was found to inhibit platelet aggregation induced by PGH2 and is about 1/10 as potent a stimulator of cAMP accumulation as is PGE1. The mass spectra of the methyl esters of both PGH1 and PGH2 were obtained, as were the infrared spectra of the two compounds. The nuclear magnetic resonance spectrum of PGH2 is characterized by signals at 4.58 delta and 4.47 delta for the C-9 and C-11 protons, respectively.