We have studied concentration of C-reactive protein (CRP), alpha-1-acid glycoprotein (AGP) and alpha-1-antychymotripsin (ACT) and their major microheterogeneity (AGP-RC, ACT-RC) in 46 patients during lithium prophylaxis: 21 – up to 2 years, 25 – over 10 years and in 20 healthy controls. Concentrations of CRP were in the range of healthy controls in both groups. AGP and ACT levels were in both groups higher than in controls. The values of microheterogeneity of ACT and AG P were elevated in the patients treated for 2 years and the patients treated for more than 10 years were in the range of controls. The results obtained may suggest that during prophylaxis lithium exerts the immunomodulatory effect on glycosylation of acute phase proteins.