This document specifies general rules to determine the concentration of microorganisms present in plant biostimulants. The method is applicable to microbial plant biostimulants for verifying that the concentration of microorganisms does not exceed the respective limits outlined in the EU Regulation on Fertilising Products [1]. This horizontal method might not be appropriate in every detail for certain products. In this case, it is necessary to refer to the methodology of specific determination and quantification of the microorganisms.