The aim of this work was to study the interactions of methyl-parathion (MP) with human (HSA), bovine (BSA) and fish (FSA) albumins by using the fluorescence quenching technique. The fish species used was the pacu (Piaractus mesopotamicus), a typical inhabitant of Brazilian rivers. MP is a pesticide still used in agriculture and fish hatcheries in many countries. Methods: MP was purified from a commercial grade preparation by thin layer chromatography, using dichloromethane as the mobile phase. HSA and BSA were purchased from chemical company. FSA was isolated from the pacu serum by affinity chromatography. We excited the intrinsic fluorescence of the tryptophan in albumins and observed the quenching by titrating the protein solutions with MP. The Stern-Volmer graphs were plotted and the quenching constants were evaluated. Results: The titration of HSA and BSA at 25?C produced linear Stern-Volmer plots. At 37oC, the plot is still linear for HSA, but not for BSA. The titration of pacu albumin by the pesticide at 20?C, 25?C and 30?C showed relevant slope deviations by the temperature changes. This behavior of Stern-Volmer plot of pacu albumin is analogous to the BSA. Examining the influence of the temperature at low MP concentrations on the plots, we found the occurrence of static quenching for the three albumins. Conclusions: (1) The three studied albumins interact with MP to form complexes. (2) The pprimary binding sites for the pesticide on HSA and BSA are close to tryptophan residues 214 and 212, respectively. (3) Estimated association constants for HSA and BSA were, respectively: 3.07x104 (?1.2x103) M-1 and 1.96x104 (?4.5x102) M-1 at 25?C; 1.08x104 (?2.0x102) M-1 and 8.16x103 (?1.9x102) M-1 at 37?C. (4) For pacu albumin, the Stern-Volmer constants were 1,19x104 (?3,4x102) M-1 at 20?C, 9,73x103 (?4,9x102) M-1 at 25?C, 9,37x103 (?4,4x102) M-1 at 30?C. Small

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