Peripheral blood mononuclear cells from patients with clinically active Behçet's disease or juvenile chronic arthritis (JCA) gave high responses to phytohaemagglutinin (PHA) in vitro prior to prednisolone therapy and low responses when taking prednisolone in comparison with cells from control individuals. Such effects were not significantly different at the six concentrations of cells or the four times in culture studied. Differences from controls were also seen with cells from clinically controlled, prednisolone treated Behçet's patients; the cells responded better than controls at low cell concentrations and early times in culture, the reverse occurring at higher concentrations and later times. This pattern of response suggests the presence of a higher than normal proportion of responsive cells in these patients. These studies suggest that not only is a still wider range of culture conditions required to assess fully the differences in lymphocyte function related to disease activity and prednisolone treatment in JCA or Behçet's, but also that the use of a narrow set of 'optimized' conditions may be misleading.