Muramidase lysosomes are products of cell activity in the monocyte-macrophages system, and their importance in malignant haemopathies of monocyte-macrophages origin was established years ago. The serum activity of muramidase was investigated by spectrophotometry, detecting the level of bacteriolysis account to the decrease of density of the appropriate bacterial opaque suspension under the influence of serum lysosymes. Serum muramidase activity was determined in 18 boys and 10 girls at the time when diagnosis of Langerhans cell Hystiocytosis (LCH) was established, one month after implementation of initial therapy and on three-month check-up. Disease classification stages were made by M.E. Osband criteria. Stage I had 52% of patients, stage II 38%, stage III 5% and stage IV 4% of all LCH diagnosed patients. In stage I patients serum muramidase levels were 15% higher than normal levels (our normal levels were 4 ± 1.2mg). In stage II and III serum level were two times higher than maximal normal level, and in stage IV they were three times higher than maximal normal level. In remission serum level normalized. Before clinically assessed relapse, serum muramidase activity have risen and in 2 patients who were resistant to applied therapy, serum muramidase levels had mild fluctuations at higher levels until lethal outcome. Our results show the importance of determination of serum muramidase levels in diagnosis, follow up in LCH, and is a useful predictor factor for relapse of LCH. This determination is useful in biological follow up of applied therapy efficiency.