The mutual friction between 3He and 4He II below 150 mK has been studied. Empirical relations for the adiabatic and the nonadiabatic flow properties of 3He moving through 4He have been determined using a dilution refrigerator with a single mixing chamber. The validity of the relations is verified by osmotic-pressure measurements and by measuring the properties of a double-mixing-chamber system. It is shown that superleak shunts have a strong effect on the flow characteristics. From the nonadiabatic flow properties an expression is derived for the mutual-friction-force density between 3He and 4He II. This has a strong resemblance to the Gorter-Mellink mutual-friction-force density between the normal and the superfluid components in pure 4He II. It is speculated that the 3He flow in our systems generates a 4He vortex tangle, which leads to the observed mutual friction between 3He and 4He and also to a strong clamping of the 4He to the walls.