It was previously shown that protein aggregation induced by gamma-irradiation can be prevented by L-tyrosine, even when irradiation was done in solution at doses as high as 10 kGy. It is now reported that caeruloplasmin irradiated at low to moderate doses appears more resistant to trypsin-mediated proteolysis than native caeruloplasmin. Ceruloplasmin presents an irradiation-dose-dependent decrease of oxidase activity and a slight increase of ferroxidase activity at low irradiation doses (2 kGy), followed by a decrease at high doses (4-8 kGy). In all cases, the catalytic activities are higher when caeruloplasmin is irradiated under tyrosine protection. Irradiated caeruloplasmin in the presence of tyrosine preserves its oxidase and ferroxidase activities. Caeruloplasmin irradiated in solution at a dose of 2-3 kGy and higher, in the presence and absence of L-tyrosine, remained sterile during at least 8 weeks of storage at 4 degrees C, as evaluated by the plate-count-agar method for assessing total viable lactic acid bacteria content after incubation for 48 h at 30 degrees C.