PGD2 undergoes dehydration in aqueous media to form PGJ2 and in the presence of albumin they convert to delta 12-PGJ2. Delta 12-PGJ2 is one of the ultimate metabolites of PGD2 in human body fluids and is excreted in urine, approximately 152 ng/24 h for males and 66 ng/24 h for females. Delta 12-PGJ2 and also PGA2 are actively incorporated into cells and transferred to the nuclei. Both PGs lead to cell cycle arrest at G1 phase and inhibit viral replication at non-toxic doses. Several genes including myc oncogene family expressions are suppressed and several specific proteins, mainly the heat-shock protein family, are induced prior to G1 arrest or to inhibition of viral replication. Thus cyclopentenone PGs, i.e., PGJ and A series containing a reactive alpha, beta-unsaturated carbonyl constitute a different PG category from other classical PGs. Recently other biological activities of delta 12-PGJ2 such as immunomodulation and stimulated osteogenesis have been found. The author reviews PGJ2 research and introduces several new aspects.