Since the phospholipase B (PLB) was reported as a deacylase of both lecithin and lysolecithin yielding fatty acids and glycerophosphocholine (GPC), there was a question as to whether it is a single enzyme or a mixture of a phospholipase A₂ (PLA₂) and a lysophospholipase (LPL). We purified the PLB in Penicillium notatum and showed that it catalyzed deacylation of sn-1 and sn-2 fatty acids of 1,2-diacylphospholipids and also sn-1 or sn-2 fatty acids of 1- or 2-monoacylphospholipids (lysophospholipids). Further, it also has a monoacyllipase activity. The purified PLB is a glycoprotein with m.w. of 91,300. The sugar moiety is M₉ only and the protein moiety consists of 603 amino acids. PLB, different from PLA₂, shows other enzymatic activities, such as transacylase, lipase and acylesterase. PLB activity is influenced by various substances, e.g. detergents, deoxycholate, diethylether, Fe³⁺, and endogenous protease. Therefore, PLB might have broader roles than PLA₂ in vivo. The database shows an extensive sequence similarity between P. notatum PLB and fungal PLB, cPLA₂ and patatin, suggesting a homologous relationship. The catalytic triad of cPLA₂, Ser, Asp and Arg, is also present in P. notatum PLB. Other related PLBs, PLB/Lipases are discussed.