Abstract: A marking assembly is disclosed for making reference marks on a microscope slide. The marking assembly includes a mounting ring having two clamping flats and an extensible clampbar for compressive attachment to a microscope lens objective. Replaceable ink cartridge having a marking nib mounted near its distal end is inserted in a plunger mechanism that may be selectively activated to cause the nib to mark the slide. The plunger mechanism is pivotably mounted to the mounting ring by two orthogonally oriented pivot pins which allow the plunger mechanism and nib to be selectively positioned about both vertical and horizontal axes so that the reference mark can be selectively positioned on the microscope slide. A plurality of calibration screws and springs are provided to position and lock the plunger mechanism in place when the desired vertical and horizontal positions have been selected. The plunger mechanism may be manipulated to vary the size of the reference mark.