A study of the colony forming ability of 99 normal human bone marrows using the semi-solid agar technique is presented. Feeder layers of 10(6) normal human peripheral white blood cells were used as the stimulus, and were overlaid with 2 X 10(5) bone marrow cells. The presence of human plasma either in feeder layers or bone marrow overlays inhibited colony formation when compared with studies performed with cells washed free of plasma. The number of colonies grown varied, but not significantly when the same marrow was grown on feeder layers from different donors. The inhibitory effect of normal human plasma was also demonstrated when unwashed and washed bone marrow cells were plated over feeder layers with no stimulus. Further confirmation of the inhibition by normal human plasma was made in 10 cases by the addition of 0.1 ml plasma to cultures of washed marrow over washed feeder layers. These data suggest that optimum results from in vitro culture of normal human bone marrow can be obtained only when all plasma is washed from both the feeder layer cells and the marrow cells before culturing.