When tuberculin-sensitive peritoneal exudate cells are incubated in a culture flask with tuberculin purified protein derivative, macrophage inhibition factor and other lymphokines are released into the culture medium. We have described how, if incubation is carried out in a stationary conical culture tube, intercellular contact between the peritoneal exudate cells is facilitated as the cells sediment into a pellicle at the bottom of the tube. This results in augmented release of inhibitory lymphokines into the supernatant culture medium with titers up to 10
9
times greater than those obtained by conventional culture methods using a flatbottomed culture dish or flask. When such high-titered inhibitory supernatants were subjected to fractionation by sequential Amicon ultrafiltration, two clearly distinct macrophage-inhibitory lymphokines were found. The first was present, after fractionation, in a titer of 10
12
, had a molecular weight in the range of 50,000 to 100,000, and was heat stable at 56°C for 1 h. This moiety is probably identical to guinea pig macrophage inhibition factor. Unexpectedly, a second heat-labile inhibitory substance with a molecular weight between 500 and 1,000 was found in a titer of 10
4
after fractionation. This low-molecular-weight, heat-labile material may represent a new lymphokine with a direct inhibitory action on macrophage migration. Theoretically, the data are also consistent with the possibility that it could act as a chemical immunotransmitter which stimulates amplified production of macrophage inhibition factor by lymphocytes within the cell pellicle and leads indirectly to inhibition of macrophage migration.