The amino acid 2,6-diaminopimelic acid (DAPA) is a unique component in bacterial cell walls, thus it can be used as an indirect and specific indicator for the measurement of bacterial mass content in such biological materials as feeds or digestive tract contents in ruminants. Further, DAPA's analogues 2,6-diamino-3-hydroxypimelic acid, together with some diaminopimelic acid, occurs in the cell-wall mucopeptide of certain Actinomycetales.