Apparatus for making serial dilutions in longitudinal titration trays having equally spaced transverse rows of cups, providing an elevator vertically movable into upper and lower positions, a carriage for removably supporting and locating a tray thereon and being supported and guided on the elevator for horizontal movement lengthwise of a located tray, fixed oscillatory loops above the carriage and arranged for their alignment with the cups in any row in a located tray on movement of the carriage on the elevator in its lower position, and for their reach into the aligned cups on movement of the elevator into its upper position, and operating mechanism, including controls, performing in a presettable number of cycles in each of which the elevator is raised to its upper position, the loops are oscillated, the elevator is moved to its lower position, and the carriage is indexed to bring a next cup row in a tray thereon into alignment with the loops.