Materials and Methods.-Cell cloning: Thecells used for cloning were from the spleens of normal 60-S80-day-old malesfrom the inbred mouse strain SWR, or from embryo liverstaken at about the 17th-19th day of gestation from an inbred strain of Swiss mice. The adult spleens were washed in phosphate-buffered-saline (PBS), teasedwith forceps to make a cell suspension, washedwith PBS, and then diluted in PBS to an appropriate cell numberfor cloning. The embryo liverswere made into a cell suspension with scissors and light pipetting in serum-free Eagle's medium with a fourfold concentration of amino acids and vitamins (EM). This suspension was then diluted in EM after 5 min standing to allow aggregates to sediment, and a sample of the cells used for cloning was counted in eosin solution at a final eosin dilution of 1: 2000. Cells were cloned in 50-mm plastic Petri dishes (Falcon plastics) in soft agar on a harder agar base as described previously. 1 2 EM supplemented with 20% inactivated (560C for 30 min) horse serum was used as the medium for both agar layers. Liver cells were normally seededfor cloning at 3 X 104 cells per plate and spleen cells at 1 X 106 cells per plate. Colonies were counted microscopically after the seeding of embryo liver cells, and on a bacterial colony counter with a X2 magnifying glass after seeding of adult spleen cells. In the microscopic scoring, more than 50 cells were counted as a colony. Each point in an experiment was based on the results from 2-4 plates. The feeder cells, whenpresent, were underneath the lower agar layer. Conditioned medium was added to the lower agar layer, and dilutions of conditioned medium were made so that there was always the same percentage of new EM plus 20% horse serum.

Cells used as feederlayers and for the production ofconditioned medium: Cultures of cells used as feeder layers or forthe production of conditioned medium were grown in EM supplemented with 10% inactivated horse serum. Unless otherwise stated, the normal cells were obtained from SWR or Swiss mice; secondary embryo cultures were seeded at2 X 105 cells per 50-mm plate; kidneys from 6-10-day-old mice at one kidney per plate and from 30-40-day-old mice at 2.5 X 105 cells per plate; adult spleen at 5 X 107 cells; and peritoneal cells at 1-2 X 106 cells per plate. Peritoneal macrophages were obtained from 2-3-month-old SWR males by intraperitoneal inoculation of 5 ml PBS at 4 days after intraperitoneal inoculation of 3 ml thioglycolate medium. 3 The peritoneal cells were washed twice in PBS before seeding, and the macrophages used as feeder layers after 1-2 days in culture. A mouse cellline established in vitro from Swiss embryo cells, which will be referred to as El, was also tested forinductive capacity. Unless otherwise stated, conditioned medium was used after centrifugation at 650 g for 10-15 min, and was stored before use at 4 C 4A8