Abstract: An arrangement for mixing fluids and for growing crystals by the vapor diffusion method under conditions of weightlessness has an exterior housing containing a mixing part and a reaction part. The mixing part has two cylinders in which the starting fluids are stored until the reaction is to be performed. Each cylinder has a piston associated therewith, and internal bores permit the starting fluids to be pumped between the respective cylinders to mix them. Following mixing, the fluids are pumped through a central bore to a cylinder in the reaction part, which also has a piston associated therewith. Withdrawal of the cylinder relative to the piston causes the formation of a drop, permitting the crystallization to take place. Following completion of the reaction, the cylinder is returned to its original position, sucking the fluid into it.