Abstract: A method of measuring the number of eumycete cells in a sample which comprises preparing a solution or suspension containing a sample of a medicine, food, drink, cosmetic or water, adding to said solution or suspension a 7-amino-4-methyl-coumarin derivative represented by formula (1): ##STR1## wherein R is an alkyl group, an allyl group, an aralkyl group, or a heterocyclic group, or R--CO-- is an amino acid or peptide residue, said derivative not inhibiting the hydrolysis of the amide bond of formula (1) by microorganism hydrolases contained in the samples; and measuring the fluorescence of 7-amino-4-methyl-courmarin released by the microorganism hydrolases.