The influence of serum on the motility of normal monocytes in mononuclear cell preparations was investigated using a modified Boyden assay. Undiluted sera from 9 patients with active SLE and from 6 healthy controls were studied, using pooled normal serum as a reference. When the cells were suspended in SLE serum, both spontaneous motility, and motility in response to zymosan treated reference serum, were reduced. By contrast, the capacity of the SLE sera to generate chemotactic activity on zymosan treatment was hardly affected at all. In zymosan treated control sera, pronounced spreading and decreased motility of monocytes were observed, while zymosan treatment of SLE serum promoted spreading and increased the motility of cells. No consistent relationship was found between complement components and C1q binding immune complexes and the decreased monocyte motility in the SLE sera tested. Cell motility in the assay remained unaffected by the addition of prednisolone to the reference serum.