The study was conducted to investigate the effectiveness of ultraviolet (UV) irradiation for inactivating coliform bacterial indicators in liquid swine manure samples using a laboratory-scale, low-pressure UV collimated beam apparatus. Raw liquid swine manure was brought from the field and stored at 4 'C for preliminary settling for 24 hours, which was followed by alum treatment in a jar test apparatus. Both pre-settled and alum-treated supernatants were filtered separately through different pore-sized polycarbonate membrane filters. The unfiltered and filtered pre-settled and alum-treated supernatant samples were exposed to a range of UV fluences (i.e., doses) to determine the fluence-response relationship. After preliminary settling, alum treatment, and membrane filtration, UV absorbance values of undiluted swine manure samples at 254 nm varied between 4.0 and 4.2 cm(-1) with total suspended solids concentrations of 800 to 1,800 mg l(-1). Total coliforms were reduced by 2 - to 2.5-log10 at UV fluences of 80 to 100 mJ cm(-2). Relatively small log reductions (< 0.5-log10) were observed at lower fluences (below 20 mJ cm(-2) ) in undiluted manure samples. To reduce the UV absorbance of liquid swine manure, raw manure was diluted 1:50 and 1:10 with deionized water in subsequent experiments. Almost complete inactivation (> 4 to 5 log10) was achieved at UV fluences of 20 mJ cm(-2) or higher in the swine manure samples diluted to 1:50. Based on these findings, it was proposed that UV inactivation of coliform in the undiluted liquid swine manure was limited by association of bacterial cells with particulate matter of less than 10 microm in size. Dilution may have disrupted and reduced the association between the bacteria and this fine particulate matter resulting in better dispersion and more complete inactivation by UV.