A flow injection capacitive biosensor system to detect trace amounts DNA has been developed based on the affinity binding between immobilized histone and DNA. Histones from calf thymus and shrimp were immobilized on gold electrodes covered with self-assembled monolayer (SAM) of thioctic acid. Each of these histones was used to detect DNA from calf thymus, shrimp and Escherichia coli. The studies indicated that histones can bind better with DNA from the same source and give higher sensitivity than the binding with DNA from different sources. Under optimum conditions, both histones from calf thymus and shrimp provided the same lower detection limit of 10(-5) ng l(-1) for DNA from different sources, i.e., calf thymus, shrimp and E. coli. The standard curve for the affinity reaction between calf thymus histone and DNA shows sigmoidal behavior and two linear ranges, 10(-5) to 10(-2) ng l(-1) and 10(-1) to 10(2) ng l(-1), could be obtained. The immobilized histones were stable and after regeneration good reproducibility of the signal could be obtained up to 43 times with a %R.S.D. of 3.1. When applied to analyze residual DNA in crude protein extracted from white shrimp recoveries were obtained between 80% and 116%.