Developmental aspects of monocyte and neutrophil chemotaxis were evaluated by the agarose and the Boyden chamber methods. Simultaneous measurements of both the distances and the cell counts were carried out by the agarose method. Before 6 years of age monocyte chemotaxis values, determined by the Boyden chamber method, were significantly lower than those of adults. The difference between adults and children were more marked before the age of 1 year (56 +/- 14 microns N = 48 in 0-1 year, 85 +/- 17 microns N = 25 in adults P less than 0.0001). Neutrophil chemotaxis, determined by the agarose method, was significantly lower than adults in newborns. Values increased gradually thereafter and reached to adult levels after the age period of 2-5 years. Before 6 years of age chemotactic indices (chemotaxis/random migration) for both cell count and distance were also significantly lower than adults. In 20 individuals, neutrophil chemotaxis was assessed simultaneously by both the agarose and the Boyden chamber methods and showed good correlation in reference to distance measurements (P less than 0.01). Cell count and distance measurements, by the agarose method, were also well correlated (P less than 0.01) but agarose method and cell count determination seemed to be most sensitive. Distance/cell count ratio was found to be significantly higher in newborns (1.61 +/- 0.45) when compared with adults (1.13 +/- 0.16) (P less than 0.001). Use of age-matched controls in the evaluation of chemotaxis in children, particularly under 5 years of age, is imperative.