a substituted monoethylglycinexylidide or analogue is disclosed . the xylidide or analogue has the structure of fig . 1 of the attached drawings where m is ch 2 nhch 2 ch 3 , ch 2 ch 2 ch 2 ch 3 or ch 2 och 2 ch 3 , one of y 1 and y 2 is h and the other is a protein or a fluorescein moiety chemcially bonded to the glycinexylidide or analogue moiety . also disclosed is a method for carrying out immunoassays for megx , which has the structure of fig . 3 of the attached drawings . the method for carrying out immunoassays involves using the foregoing compounds as tracers and immunogens .