A biological reaction layer of a dry analytical element in the form of a composite porous layer in which a particulate material carrying a biologically active material fixed thereon is dispersed in a fibrous porous matrix, characterized in that the dry weight ratio of said particulate material to said fibrous material contained in said composite porous layer ranges from 1:20 to 1:0.3 and said particulate material is contained therein in the range of 1 to 60 g/m.sup.2.