Studies of the possible viral etiology of human leukemia have required large quantities of cultured cells derived from human hematopoietic tissues. Since cultures sufficiently large and free from contamination could not readily be produced according to existing methods, a pilot cell culture plant has been constructed for the production of mammalian cells in mass quantity. 500-ml to 20-liter trophocell units have already proved to be scientifically and economically practical, as they provide good reliability, excellent growth rates, and sustained yield cf human cells. 200-liter stainless steel culture units have now been added to the trophocell system. Five complete 200 liter units are now in operation. The design of the original stainless steel unit was based on that of a stainless steel, jacketed soup kettle. There are no openings in the vessel other than those in the lid, which provide convenient access points for sampling, sensor probes, etc. Environmental parameters, e.g., liquid level, temperature, and pH, are monitored and controlled with commercially available apparatus. Many initial problems connected with the new 200 liter units have been resolved, but operational and design problems remain in the areas of stable instrumentation, cell harvesting, salvaging and reuse of unspent media components, establishment of physiologic steady state, recovery of virus-containing cells with reculture of the remaining unaffected cells, and the recovery and separation of cell components and special products such as immunoglobulins, interferons, and hormones. A definitive cell plant with culture units of 20,50,250, and 1250 liters is now being constructed.