Effect of MS-5 cells on the number of CFC produced in each clone at week 5. Single cells were cultured as described in the legend of Fig 3, either in the presence (•) or the absence (○) of stromal cells. After 10 days, each clone was replated in standard LTC-IC assays and the number of LTC-IC–derived CFC assessed at week 5. Each point represents the CFC output per individual clone seeded either with (•) or without (○) MS-5 during the first 10 days of culture. For each condition, the geometric mean of the numbers of CFC produced is indicated by the horizontal bar.