The microbial matter fraction was determined in perennial ryegrass silages of different dry-matter (DM) contents, ensiled with or without Lactobacillus plantarum. 15N-Leucine and the bacterial cell wall constituent diaminopimelic acid (DAPA) were used as markers for microbial-N. Perennial ryegrass crops with DM contents of 202, 280 or 366 g kg-1 fresh weight were ensiled in laboratory-scale silos and stored for 3 to 4 months. At different times after ensiling, silages were analysed and microbial fractions were isolated. Microbial-N concentration determined with 15N-leucine reached a maximum during the first week of ensilage. It remained unchanged thereafter, except in silage with a DM content of 280 g kg-1 in which it decreased (P < 0.01) by 32uring storage. After 3 to 4 months ensilage, microbial-N concentration varied from ≅0.3 to ≅1.7 g kg-1 DM. A negative relationship was observed between microbial-N concentration and silage DM content. Inoculation resulted in an approximately twofold increase (P < 0.001) in microbial-N concentration. Microbial-N concentrations determined with DAPA were 1.14-2.07 times higher than those determined with 15N-leucine. However, 19-35␘f the DAPA in silage occurred in a soluble form, indicating that this fraction of DAPA was not associated with intact bacteria.