The effects of phorbol esters on shape change and locomotion of human blood lymphocytes were studied both immediately after separating the cells from blood and after overnight culture. Phorbol myristate acetate (PMA), phorbol dibutyrate (PDB) and related esters produced complex shape changes in lymphocytes at both times. These shapes were analysed quantitatively using objective measurements derived from the moments of cell shapes. Immediately after removal from blood, many lymphocytes (20–60% of the total) protruded and retracted veils or spikes at more than one point on the cell surface. The morphology of these cells was not typical of locomotor cells. Usually, formation of a veil was not followed by a contraction wave moving down the cell, though some cells did show contraction waves, and some moved into collagen gels or filters. After overnight culture, a high proportion (70–80%) of cells had changed shape in PMA and PDB. Although the shapes were still atypical, they resembled classical locomotor morphology more closely; veils formed at one point on the cell surface tended to persist, and contraction waves and constriction rings were seen in many cells. These cells moved in large numbers into collagen gels or filters. Comparison of the paths traversed by PMA-treated lymphocytes in collagen gels suggested that cells cultured in PMA for 24 h pursued more persistent paths that those in short-term culture, but the difference was not marked. We suggest that phorbol esters induce immediate shape change without inducing the complete sequence of motor events necessary for efficient locomotion, whereas after prolonged culture in phorbol esters, locomotion is more efficient, possibly because phorbol esters, like other growth activators, stimulate events during the G1 phase of growth that are necessary for full expression of locomotor capacity.

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