Piroxicam (PXM; a non-steroidal anti-inflammatory drug) has been reported to induce photosensitivity. In our previous report, however, ultraviolet-A (UVA)-irradiated or non-irradiated PXM did not induce any reactions in the in vivo model of contact hypersensitivity, while positive patch testing was shown by ampiroxicam (APX; a prodrug of PXM). The purpose of the present study was to clarify the influence of protein on the antigenicity of PXM using this model. Animals sensitized by UVA-irradiated 1% APX showed positive patch testing (open application) in UVA-irradiated 1% APX, while they were negative in challenge by UVA-irradiated PXM with or without 5% human serum albumin (HSA). Although animals sensitized by 1% thiosalicylate (TOS), which is thought to be an active hapten of PXM, were cross-reacted with UVA-irradiated 1% APX, they failed to react with UVA-irradiated 1% PXM with or without HSA. On the other hand, intra-dermal testing (intra-dermal application) in UVA-irradiated 0.1% PXM with 5% HSA was positive in animals sensitized by UVA-irradiated 1% APX, while 5% HSA alone, 0.1% PXM with 5% HSA and UVA-irradiated 0.1% PXM did not induce any reactions under this condition. Furthermore, concentration of PXM in the presence of HSA was reduced by UVA-irradiation in a time dependent manner, while the degradation of PXM was not observed in the absence of HSA. Finally, PXM almost disappeared at 120 min after the initiation of UVA-irradiation. The degradation of PXM irradiated by UVA was dependent on the concentration of HSA at the range of 0 to 4%. Hence, these results suggest that the presence of protein is necessary for the induction of the antigenic activity of PXM and the antigenic characterization of PXM is different from that of APX in contact hypersensitivity.