density gradient in the appropriate ranges prior art solutes for such solutions include e g sucrose potassium bromide cesium chloride potassium tartrate and the like the isopycnic banding step is conveniently carried out in a centrifuge for example electronucleonics k by filling the stationary rotor with saline solution then successively displacing the saline solution upwards with aliquots of a liquid medium solution of increasing density until a step gradient is formed the plasma is introduced at the top of the rotor displacing some of the highest density solution from the bottom typically the volume of plasma is from about 15 to about 40 that of the step gradient the centrifuge is brought up to speed through a programmed speed control system which prevents mixing during the initial reorientation phase when equilibrium is attained and the product is in its power density position the rotor is slowed down through the same system to prevent mixing upon reorientation to the original configuration then the gradient is drained from below and the proper density cut collected a similar technique is used in the rate zonal bonding the proper density cut from the rate zonal banding is the desired concentrate of hepatitis b antigen due to the small size approximately 20 nm of hb s ag the isopycnic banding step is quite time consuming requiring about 18 hours of centrifuging as a result even operating 24 hours a day 7 days a week it is possible to prepare only about 4 batches of clarified plasma per centrifuge productivitiy can be increased of course by utilizing additional centrifuges this involves a tremendous capital investment however as each centrifuge.