Association of 7-methylguanosine 5'-monophosphate with a tryptophan containing tripeptide, Trp-Leu-Glu, has been studied by fluorescence titration using two different geometries of detection, viz. right angle and front surface geometry. The applicability of these two techniques to determine the stability constant of the nucleotide-peptide adduct is discussed. Evidence is presented that fluorescence titration based on right angle detection may lead to considerable overestimation of the strength of interaction.