A principal problem facing human DNA studies that use old and degraded remains is contamination from other sources of human DNA. In this study we have attempted to contaminate deliberately bones and teeth sampled from a medieval collection excavated in Trondheim, Norway, in order to investigate this poorly understood phenomenon. Five pairs of teeth and bone samples were bathed in water containing various concentrations (from 10-9 and 10-21 g/l) of purified ?X174 DNA. Subsequently the samples were subjected to a routine decontamination protocol involving a bleach bath followed by exposure to ?=254 nm ultraviolet light, prior to DNA extraction and analysis for evidence of the persistence of the contaminant. The results support previous speculation that bone is more susceptible to water-borne sources of contaminant DNA, although both bone and teeth are readily contaminated and are difficult to decontaminate using the tested protocol. We believe that this is largely due to the porous nature of bone and teeth facilitating the deep penetration of the contaminant DNA. To simulate a more realistic handling situation, 27 further teeth were directly handled and washed, then decontaminated, prior to assaying for the residual presence of the handler's DNA. Surprisingly, although our results suggest that a large proportion of the teeth were contaminated with multiple sources of human DNA prior to our investigation, we were unable to contaminate the samples with further human DNA. One potential explanation may be the deposition of sediment or other structural changes that occur within the samples as they desiccate post-excavation, which may protect samples from subsequent contamination, but also prevent the efficacy of bleach baths in decontaminating specimens. Copyright © 2006 John Wiley & Sons, Ltd.