Efficiency of UV disinfection of unfiltered and filtered secondary wastewater effluent, using a large-scale pilot system, and the inactivation of six bacterial species in a laboratory UV-device have been studied. Pilot plant studies revealed low levels of coliforms and streptococci (3 logarithmic units reduction) when a wastewater UV transmission of 45% and an average effective UV dose of 100 mW a cm(-2) were used. By contrast, removal of Pseudomonas aeruginosa appeared insufficient (<1.1 logarithmic units). Laboratory investigations, using the six bacterial species as reference, showed that the kinetics of bacterial removal agreed with the model of Chick-Watson, which is formulated by the equation dN/dt = -k'(CN)-N-n. It also appeared, that according to the UV dose applied, two types of inactivation prevailed: a high rate of inactivation, with weak UV doses, and a low rate of inactivation with relatively high doses. Each examined strain was characterised by two K coefficients, (K-1 and K-2). Lower the K value, more the bacterial resistance to UV. Hierarchical classification of bacteria, based on the values of kinetic constants, revealed three groups of different sensitivity to UV. A resistant group, represented by the two strains of P. aeruginosa ATCC 15442 and Bacillus subtilis 6633, and a sensitive group, with the strain of Escherichia coli ATCC 11229. A medium group was represented by Enterococcus ATCC 19433, Serratia marcescens ATCC 8100 and P. aeruginosa S21. Kinetic studies revealed that the first instants of exposure (2-10 s) to a UV intensity of 5-8 mW cm(-2) appeared as deciding factors in the disinfection with UV irradiation. The importance of cumulative UV dose was only secondary. Photoreactivation of micro-organisms following UV disinfection was observed with all bacterial strains studied here, except for P. aeruginosa ATCC 15442, P. aeruginosa ATCC 15442 mutant mt and Enterococcus hirac ATCC 10541.