a fluid deposit assembly mounted on a carrier for depositing minute drops of fluid at selected locations upon a substrate , comprising a deposit element having an exposed tip of diameter of 0 . 3 mm or less constructed and arranged to carry and deposit drops of fluid upon the substrate , stable lateral reference surfaces or surface portions exposed for engagement by the deposit element , the surfaces or surface portions being constructed and arranged to prevent x , y displacement of the deposit element relative to the carrier when the deposit element is urged thereagainst and design for urging the deposit element against the reference surfaces or surface portions at least at the time that the deposit element approaches a substrate to deposit a fluid drop . the reference surfaces or surface portions and the design for urging are cooperating to precisely position the deposit tip in a precisely desired position as it contacts the substrate . the deposit element is shown as the tip of an axially moveable pin . the reference surface portion is shown , among others , as a surface of revolution whose axis is disposed at a predetermined position relative to the carrier . a mobile supply in the form of a multiwell plate is moved under computer control to be a close companion of the deposit device , enabling rapid deposit action and avoidance of evaporative effects that can alter the consistency of dots in the array being formed . the device can deposit one dot on top of another from fluid in a mobile multiwell plate that moves to be close to the deposit device .