The Maillard reactions between proteins and glucose or fructose, in 0.2M phosphate buffer (pH 7.4) at 37 degrees C, were investigated and the following results were obtained: 1) 3-deoxyglucosone (3DG) is formed as the major carbonyl intermediate; 2) fructose has a higher reactivity than glucose and 3DG is formed also from fructose without involvement of amino groups; 3) 3DG is a crosslinker responsible for polymerization of proteins; 4) 3DG is involved in the formation of a fluorescent advanced-stage product named "Peak L1" in vitro and in vivo.