Luteinizing hormone (LH) is a heterodimeric glycoprotein containing varied amount of sialic acid. This is a reason of numerous LH isoforms called also isohormones. The hormone isoforms were separated usually by gelelectrophoresis, isoelectrofocusing or chromatofocusing. They differ in biological and immunological activity. Human and some animals LH isoforms were reviewed. Also some genetic mutants of LH are described. Problems of the human isoforms for pathology and diagnostics are presented.