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When cultured in vitro, human lymphocytes produced a range of materials which were able to reduce the migration inhibitory effect of human macrophage migration inhibition factor (MIF) and in some cases stimulated macrophage migration to greater than control levels. In addition, lymphocytes stimulated with antigen or mitogen produced a lymphokine-like migration stimulation "factor". Detectable production of this factor occurred 24--48 h after antigen stimulation. The migration stimulation factor (MStF) (MW 50-250,000; pI 6-5) was not chemotactic and could be separated from migration inhibition and chemotactic factors by chromatography and iso-electric focussing.