Immunological methods have been widely used in clinical chemistry because of their sensitivity, specificity and practicability. Methods used for counting have been determined by the labels bound to immunoreactants. Radioistopically labeled antigens and antibodies were widely applied in this field. Although radioisotopic methods extremely sensitive and quite precise, alternative methods used enzymes, luminescent compounds, fluorescent probes as labels have been introduced. In recent years, the development of time- resolved fluorescence detection method by the use of Lanthanide chelates as labels in immunoassays has opened new possibilities given more sensitive results for the determination of biological substances. Keywords :