The aim of this study was to determine an influence of ionized radiation on the prevalence of Cryptosporidium and Pneumocystis carinii (P. c.) infections in children inhabitants of settlements affected by radionuclides after Chernobyl accident. Survey of children from day-care centers for prevalence of Cryptosporidium was carried out in Bryansk region, Russia, and Gomel and Mogilev regions, Belarus; children from cities Mogilev and Gomel as well as small town Pochep, Bryansk region, serve as control clean zones. Survey for P. c. seroprevalence was carried out in town Novozybkov, Bryansk region, Specific Ab to P. c. determined by "Pneumo-test" system (Nyarmedic. Moscow. Russia), an evaluation of immune status included assessment of Ig in serum and saliva by Manchini method. Results of the study were as follows: 1) of 432 healthy children surveyed for Cryptosporidium oocysts 352 were from affected regions: the prevalence in Bryansk region was 8% vs. 2.8% in control, in Gomel and Mogilev 4.1% vs. 2.8%. 2) Prevalence anti-P.c. Ab accounts 56.3% in children and 78.2% in adult inhabitants of Novozybkov vs. 30% in children of control group. Special interest presents an relationship of seroprevalence P.c. and secretory immunity examined twice with 2 month interval. At the first survey IgG in seropositive for P. c. children was found in 36.8% (vs. 23.1% in control) and secretary IgA in 13.2% vs. 5.8% in control. At the second survey IgG was found in 53.4% vs. 21.8% and IgA in 2.7% vs. 1.8% in control. Taking into consideration the known fact that IgG and IgA in saliva are result of their transudation from blood through the inflammatory mucosa we consider that long ionized radiation impairs the barrier function of mucosae.