Second harmonic generation (SHG) was used to selectively probe DNA drug interactions without the need for chemical labels or invasive detection methods. In particular, the binding constant of the anticancer drug daunomycin to a recognition triplet sequence in a 33-mer of double stranded DNA was determined. The SHG method, which is interface selective, probed the binding of daunomycin to DNA that was tethered to the surface of colloidal microparticles suspended in aqueous solution. Probing biomolecule coated colloids is expected to yield larger SH signals and provides experimental flexibility as compared to experiments performed at planar interfaces. The change in SHG intensity as daunomycin was added to the microparticle solution was fit to a Langmuir binding model, which yielded an equilibrium constant of 2.3 (+/- 0.7) x 10(5) M-1; the corresponding Gibbs free energy change at 20 degrees C is -7.2 +/- 0.2 kcal/mol. Control experiments established that daunomycin preferentially binds to DNA at the recognition sequence. The equilibrium was found to be unaffected by the presence of free DNA in solution, and hyper-Rayleigh scattering from bulk molecules did not change with increasing daunomycin concentration. The extracted equilibrium constants are in agreement with the range of reported values found in the literature.