Laboratory studies of the transmission and pathogenesis of Renlbactenurn salmonmarum may descnbe more accurately what is occurring in the natural environment if test fish are infected by waterborne R salmoninarum shed from Infected flsh To quantify bacterial sheddlng by chinook salmon Oncorhynchus tschawytscha at 13°C in freshwater, groups of fish were injected mtrapentoneally wlth R salmoninarum at either 1 3 X 106 colony formlng unlts (('FU) fish-' (high-dose injection group) or 1 5 X 103 CFU fish (low-dose ~nlection group) R salmoninarum Infection levels were measured in the exposed flsh by the enzyme-l~nked imrnunosorbent assay (BKD-ELISA) At regular intervals for 30 d , the numbers of R salmoninarum shed by the injected fish were calculated on the basis of testlng water samples by the membrane f~ltration-fluorescent ant~body test (MF-FAT) and bacteriological culture Mean BKD-ELISA optical dens~ties (ODs) for flsh in the low-dose injection group were not different from those of control fish ( p > 0 05) and no R salmon~narum were detected in water samples taken up to 30 d after inlection of fish in the low-dose group By 12 d after inlection a proportion of the flsh from the high-dose Infection group had high (BKD-ELISA OD 2 1000) to severe (BKD-ELISA OD 2 2 000) R salmonlnarurn infection levels, and bactena were detected In the water by both tests However, measurable levels of R salmonlnarum were not consistently detected in the water u n t~l a proportion of the fish maintained high to severe infection levels for an additional 8 d The concentrations of R salmoninarum in the water samples ranged from undetectable up to 994 cells ml-' on the b a s~s of the MF-FAT, and up to 1850 CFU m1 ' on the basis of bactenological cultureThe results suggest that chinook salmon infected with R salmonlnarum by inlection of approx~mately 1 X lob CFU fish-' can be used as the source of infection In cohabitat~on challenges beginning 20 d after ~nlection