An adjustable pipette (10) includes a body (12) containing a plunger assembly (14) including a thumb button (22) that is axially moved in alternating relatively shorter sample aspirating and relatively longer dispensing strokes. Guide slots (108) in a guide sleeve (32) receive a stop pin (52) carried by a plunger shaft (48), and the slots (108) define two axially and circumferentially spaced lower stops (104, 106) for aspirating and dispensing strokes respectively. The thumb button (22) includes offset lobes (112, 114) that apply a rotational torque to the plunger shaft (48) when pressed by the user. The user presses one lobe (112) for an aspirating stroke to rotate the plunger shaft (48) in one direction and align the stop pin (52) with uppermost lower stop (104) for a relatively short aspirating stroke and the user presses the other lobe (114) for a dispensing stroke to rotate the plunger shaft (48) in the other direction and align the stop pin (52) with lowermost lower stop (106) for a relatively long dispensing stroke. A window (116) in the body permits the user to view and thereby understand the function of the lower stop selecting mechanism.