The "indentation size effect" described in some reports of ultra-micro-indentation hardness tests has been examined to elucidate its cause. The magnitude of the effect was found to be associated with the strain-hardening propensity of the material tested, and thus similar to that observed in conventional tests with greater force, but the ranges of hardness variation were discontinuous. The relatively small size effect in ultra-micro-indentation testing seems to imply very low effective friction between indenter and specimen : a condition which could be due to ’dither’ in the closed-loop servo control of ultra-micro-indentation.