An investigation was made of the capacities of humans to detect, by actively touching with the fingertip, the presence of a single, small asperity on a very smooth background. The asperity consisted of either a raised dot having a diameter of 602, 231, or 40 micron, or an edge, each etched into a silicon wafer using the methods of contact photolithography. The height of each dot or edge was varied and the subject was asked to make a forced choice on each test trial as to which of two wafers, one of which was blank, contained the asperity. The mean detection threshold, or minimal height of asperity corresponding to a d' of 1.35, was lowest for edges (0.85 +/- 0.22 micron, SD) and increased with decreases in the diameter of dot from 1.09 +/- 0.19 micron for a diameter of 602 micron to 2.94 +/- 1.19 micron and 5.97 +/- 2.02 micron for diameters of 231 micron and 40 micron, respectively. The type of skin displacement required for the detection of these small asperities was believed to be a local lateral deformation of the papillary ridges.