Abstract: An enzyme immunoassay for an antigen using a solid phase, which employs (a) a biotinylated antibody in which a biotin derivative is bound to a thiol group of an Fab' fragment of an antibody, (b) an enzyme-labelled antibody, and (c) a solid phase immobilized with a substance capable of specifically reacting with the above biotin derivative selected from the group consisting of avidin, streptoavidin and a derivative thereof which is bound to the solid phase directly or via a linkage between another biotin derivative and a high molecular weight substance bound to the solid phase, or alternatively employs (a') a solid phase immobilized with a biotinylated antibody via a substance selected from the group consisting of avidin, streptoavidin and a derivative thereof wherein said substance is bound to the biotin moiety of said biotinylated antibody and is also bound to the solid phase directly or via a linkage between another biotin derivative and a high molecular weight substance bound to the solid phase, and (b') a

Abstract: Urea, creatinine, creatine, triglycerides, or the like in a specimen can be accurately determined by terminating an isocitrate dehydrogenase reaction by addition of a chelating agent in a system wherein NADP.sup.+ formed from NADPH is reproduced into NADPH in the conjoint presence of an isocitrate, metallic ions such as magnesium or manganese ions, and isocitrate dehydrogenase in assaying a substance by means of a reaction of NADPH to NADP.sup.+.

Abstract: Urea, creatinine, creatine, triglycerides, or the like in a specimen can be accurately determined by terminating an isocitrate dehydrogenase reaction by addition of ATP and/or a chelating agent in a system wherein NAD.sup.+ formed from NADH is reproduced into NADH in the conjoint presence of an isocitrate, metallic ions such as magnesium or manganese ions, and isocitrate dehydrogenase in assaying a substance by means of a reaction of NADH to NAD.sup.+.