The structures of Asn-linked sugar chains of the bullfrog (Rana catesbeiana) lutropin (LH) with different isoelectric points (pI 9.3 and pI 8.8) have been analyzed. The LH of pI 9.3, which is three times more active in Xenopus ovulation assays and radioreceptor assays with Xenopus testis than LH of pI 8.8, has Man-GlcNAc-GlcNAc and Man-Man-GlcNAc-GlcNAc as the major sugar chain components both at Asn-57 and 83 of the alpha-subunit. On the other hand, the alpha-subunit of the LH of pI 8.8 contained more mannose and phosphate bound at the nonreducing end of the sugar chains. This is the first study to identify the presence of a phosphate group bound to the mannose in LH. The structure of the major sugar chain bound to the beta-subunit was Man-Man-GlcNAc-(Fuc)GlcNAc at Asn-8 of the LH from both pIs. These sugar chains bound to the bullfrog LH are more simple than those of mammalian LH, which have N-acetyl-galactosaminylated biantennary oligosaccharides and more complicated sugar chains. The microheterogeneous pI of bullfrog LH is thus due to the different phosphate contents in the sugar chains bound to the alpha-subunit instead of sulfate groups which are found in mammalian lutropins.