In this work chemiluminescence (CL) of luminol was used for the determination of Co(II). This method is based on the fact that Co(II) catalyzes the CL reaction of luminol in the presence of H2O2. The optimization of the conditions of CL measurements in a liquid system (effect of luminol and H2O2 concentrations) is studied. This method has shown a remarkable sensitivity until 4 fg/L of Co(II) as detection limit. The linear range extends from 0.4 pg/L to 40 mu g/L of Co(II). The optimum conditions for the preparation of the chitosan membranes for the adsorption of cobalt ions were studied. The analytical characteristics of the membrane/liquid CL system are similar to those of the CL liquid system. The preconcentration of cobalt ions on the chitosan membrane presented here opens up possibilities for a number of biomedical applications for determination of Vitamin B-12 containing a cobalt ion in pharmaceuticals, human serum, egg yolk and fish tissue, and for environmental applications for the detection of cobalt in soils and river waters.