1. Impulses in single nerve fibres from the lateral antebrachial cutaneous nerve were recorded using the microneurography technique in human subjects. 2. In a sample of fifty-five mechanoreceptive units with fast-conducting nerve fibres, five types were identified, i.e. SAI (slowly adapting type I, Merkel), SAII (slowly adapting type II, Ruffini), hair units, field units and Pacinian-type units. The latter three unit types were all rapidly adapting. 3. The detailed structure of thirty-five receptive fields of SAI, SAII, hair and field units was explored with a method which was objective and independent of the experimenter's skill and experience. A lightweight probe was used to scan the receptive field area in a series of tracks 0.23 mm apart while single-unit activity was recorded. 4. SAI fields were small and composed of two to four well-separated high-sensitivity spots and often, in addition, one minor spot of lower sensitivity. SAII units typically fired spontaneously at a low and regular rate. Most fields consisted of one single spot of high sensitivity with diffuse borders. The hair units innervated ten to thirty-three (or more) hairs, which were evenly distributed over a large area. The field units were characterized by a number of small and closely packed high-sensitivity spots with diffuse borders. A conservative estimate indicated eleven spots per unit. 5. The findings indicate that the sheet of mechanoreceptors on the skin of the forearm is distinctly different from that on the dorsum of the hand and in the face. It seems reasonable to assume that the former is more representative for the hairy skin covering the main parts of the body.