The influence of interleukin-1 (IL-1) upon leucocyte locomotion in vitro was studied, using either casein or zymosan-activated serum (ZAS) as chemotaxigens. A pre-incubation of polymorphonuclear cells (PMNC) with ultrapure (purity 99%) human IL-1 (1 U/ml) for 2 and 5 h followed by a washing step resulted in a significant decrease in leucocyte locomotion against casein (P less than 0.0005 and P less than 0.01, respectively), but not against ZAS (P greater than 0.1). Moreover, the direct addition of this IL-1 preparation to leucocyte locomotion assays without pre-incubation produced a similar and significant inhibition of leucocyte locomotion directed against casein (P less than 0.05). This inhibitory effect could not be augmented further by higher concentrations of ultrapure IL-1. In order to exclude the effect of possibly contaminating cytokines in the ultrapure IL-1 preparation used, additional assays with recombinant human IL-1 (rIL-1) alpha and rIL-1 beta (1 U/ml) were performed to investigate their influence upon leucocyte locomotion. It was found that both rIL-1 alpha and rIL-1 beta inhibited leucocyte locomotion directed against casein significantly (P less than 0.005). Similar to the previous experiments, leucocyte locomotion could not be further inhibited by higher concentrations (10 U/ml) or rIL-1. Thus, both ultrapure and rIL-1 were found to have the ability to inhibit leucocyte locomotion in vitro.