Riboflavin sensitized photodynamic modifications of high molecular weight Kininogen (HMWK) isolated from sheep (Avis-arias) plasma leads to inactivation of antiproteinase activity and formation of aggregated products. A continued disappearance of the inhibitory activity towards papain and formation of high molecular weight adducts was observed with increasing concentration of riboflavin and varying time periods of incubation reaching a maximum value of over 85% (loss in activity). Aggregates resisted dissociation upon heating at 100 degrees C in 1% SDS. Aggregation and photoinactivation of HMWK was promoted by the substitution of H2O for deuterium oxide (D2O), which is known to prolong the life span of singlet oxygen, and suppressed by sodium azide a known singlet oxygen quencher. Mannitol and thiourea (hydroxyl radical scavenger) did not protect the antiproteinase activity of HMWK. Treatment with reducing agent resulted in decrease of the aggregated products suggesting the possible involvement of disulfide linkages in protein crosslinking. Tryptophan fluorescence was completely lost and significant production of dityrosine was detected in photoinactivated HMWK aggregates. Changes in the far Ultra violet circular dichroism (u.v.c.d.) spectrum of HMWK was indicative of loss of secondary structure. Analysis of modifications induced in HMWK by riboflavin reveals that the processes proceed via a singlet oxygen mediated pathway. It is concluded that the susceptibility of HMWK to oxidation may arise from oxidative modifications by reactive oxygen species generated in plasma.