Abstract: A fluid, for example, blood, specimen holder for biological testing comprises an elongate transparent tube of equilateral triangle cross section. The intersection region of two sides of the tube forms a channel for controlling, channelling, and concentrating flow of a fluid specimen as the tube is agitated to flow the fluid specimen back and forth between ends of the tube during biological fluid testing. The third side of the tube opposite the fluid flow channel provides a flat impinging surface for a light source used in determining translucency characteristics. A removable end cap for the tube is formed having a small diameter inner portion which projects into the tube when the cap is installed thereon. A transverse diaphragm provided at the inner end of the inner portion enables the cap to be penetrated by a hypodermic needle for introducing a fluid specimen into the tube. The projecting inner portion is configured for preventing escape of fluid from the tube during biological testing.