An activated sludge pilot plant was used to acclimatise sludge to a low dose of H2S gas. Sludge samples from different types of treatment works were compared with acclimatised and unaclimatised sludges using batch absorption tests. The effects of sludge source and acclimatisation on the ability of the sludge to withstand shock loads of 50 ml l(-1) H2S were evaluated. Sludge that had been acclimatised to 5 ml l(-1) H2S in the air supply removed 70% of a 50 ml l(-1) load in a batch reactor after three sludge ages of acclimatisation. Pre-exposure to low H2S concentrations enabled the bacterial community to maintain a sufficient sulphide-degrading population to retain acclimatisation and degrade subsequent high loads which are toxic to unacclimatised sludge. Acclimatisation to H2S is therefore mainly an effect of selection pressure on the mixed bacterial population, which suggests that the H2S removal capacity of different types of activated sludge will converge after acclimatisation, irrespective of their initial degradative abilities.