Vast amounts of animal manure produced from concentrated animal feeding operations have the potential to be converted into economic gain if the proper processing technology is employed. Anaerobic digestion (AD) is an effective way to convert organic wastes including animal manure into profitable by-products as well as to reduce the pollution of water, air, and soil caused by these wastes. Two-phase AD of manure offers several advantages over conventional one-phase AD. Solids reduction through hydrolysis/acidification might be very significant for animal manure which contains high amounts of solids. However, to date, studies of two-phase AD of animal manure have been limited to screened manure. Therefore, this study investigated the two-phase AD of dairy manure with particular emphasis on the effects of retention time and organic loading rate (OLR) on anaerobic acidification and biogasification of unscreened dairy manure. The results indicated that pre-acidification of dairy manure in daily-fed continuously-mixed reactors with no recycle led to significantly high reduction efficiencies of volatile solids and, thus, biogas production in the subsequent methanogenic reactor especially at OLRs of 4-10 g VS dm(-3) day(-1). However, the extent of the stimulation in the biogas production relative to corresponding feed samples was quite variable (between 6.9 and 64.7%) for different solids retention times and OLR combinations. A relatively lower performance was observed for the high OLRs (20-30 g VS dm(-3) day(-1)) used which was attributed to the possible wash-out of the acidifiers at the considerably low retention times (1.25-4 days) used. (C) 2004 Society of Chemical Industry.