Objective: To compare, in vitro, the shear bond strength (SBS) of two bond systems: Transbond XT/XT primer (TXT/XT) and Transbond Plus Color Change/Transbond Self Etching Primer (TPCC/ TSEP). Materials and Methods: Each system was examined under four enamel surface conditions (dry, water, saliva, and blood), and 160 bovine teeth were divided into eight groups of 20 according to enamel surface condition. Group 1 used TPCC/TSEP and Group 2 used TXT/XT under dry conditions; Group 3 used TPCC/TSEP and Group 4 used TXT/XT with water; Group 5 used TPCC/ TSEP and Group 6 used TXT/XT under saliva; and Group 7 used TPCC/TSEP and Group 8 used TXT/XT with blood. Brackets were bonded, and the samples were thermocycled 500 times between 5uC and 55uC; they were then submitted to a shear bond test with a universal testing machine with a 50 kgf load cell at 5 mm/min. Results: Although water and saliva affected TXT/XT more than they affected TPCC/TSEP, there were no significant differences among the groups (Groups 3 and 4: P 5 .940; Groups 3 and 5 and Groups 4 and 6: P 5 1.000; Groups 3 and 6: P 5 .988; Groups 4 and 5: P 5 .690; and Groups 5 and 6: P 5 .861). Conclusions: The presence of blood resulted in the lowest SBS from both bond systems, but especially from TXT/XT. TPCC/TSEP resulted in a higher SBS than TXT/XT under all conditions except the dry enamel surface. (Angle Orthod. 2010;80:963-967.)