Increased susceptibility to infections can be a consequence of altered function of immune cells including neutrophils. The goal of the study was to evaluate the process of neutrophil activation via Ca(2+)-mediated signal. The study was performed on isolated peripheral blood neutrophils obtained from 41 children with recurrent infections (21 girls, 20 boys) 3-10 years old with more than five episodes of respiratory tract infection (RI) per year and from a control group of 30 healthy children age and sex matched, free from allergic, immune and hematological disorders. Neutrophils were activated by bacterial peptide fMLP, opsonized zymosan (OZ), and phorbol myristat acetate (PMA). The kinetics of the intracellular Ca(2+) concentration i[Ca(2+) ] was assessed by flow cytometry (Coulter Epics XL) with the use of Fluo3 and Fura Red fluorescent dyes. Data were collected in histograms displaying Fluo3 fluorescence vs. time and Fura Red fluorescence vs. time and the mean channels of fluorescence intensity were used for calculations. fMLP and OZ-induced Ca(2+) mobilization lasted shorter in the RI group (P<0.05). The peak influx of free Ca(2+) and i[Ca(2+) ] in the resting state after stimulation with fMLP were lower in the patients (P<0.05). In the RI group stimulation with OZ was delayed compared with that in the control group (P<0.01). In response to PMA, i[Ca(2+) ] decreased faster. The kinetic slopes of i[Ca(2+) ] in both groups examined differed statistically at all points measured. A decrease in i[Ca(2+) ] after PMA stimulation was greater (P<0.01) and lasted longer in the RI group. We conclude that increased sensitivity to infections in RI children may be related to the disturbance in neutrophil activation that is mediated by changes in i[Ca(2+) ] with the subsequent production of free oxygen radicals. Such a disturbance may be inheritable or secondary to infection and antibiotic therapy.