A new method for determinating diaminopimelic acid (DAPA) from rumen fluid was developed. The concentration of DAPA is used as an indicator for the estimation of protein content of bacterial origin. Due to the performic acid oxidation preceding hydrolysis of proteins, the neighbouring amino acids do not interfere in the determination of DAPA. As a result, even trace concentrations of DAPA may be accurately determined. Since, following the performic acid oxidation the sample does not contain methionine, the buffers developed for rapid determination of methionine may be used to advantage. As a result of this, DAPA may be determined by ion exchange coílumn chromatography in ca. 18 minutes. Following the development of the analytical method, it was applied to the determination of DAPA in the rumen fluid and the bacterial proteins prepared from the rumen fluid of cattle, goats and sheep. Based on the results, a method for evaluation of protein content of bacterial origin, based on the DAPA content, was derived.