An ink jet pen supply cartridge having a spring biased ink bag with a visual indication of remaining ink quantity. The spring-bag reservoir tends to collapse laterally as the ink supply decreases due to differential pressure exerted thereto. The spring-bag is contained in a rigid cartridge and a pair of flexible tape members are cemented or welded, one to each side of the spring-bag, and extend generally parallel toward a narrow end surface of the cartridge at which they overlap and can be viewed through a window. The overlapping relationship of the tape members provide ink quantity indicia which change as the spring-bag collapse draws them past each other.