Monocyte migration in millipore filters was measured by the leading front method. The precision, sensitivity and specificity of the method was assessed. The migrational response of monocytes to N-formylmethionylleucyl-phenylalanine (FMLP) was equal in concentrations ranging from 5 X 10(-9) to 5 X 10(-7) M. No dose-response relationship was demonstrated. The random analytical variation of the method was considerable, the mean coefficient of variation of duplicate experiments being 9.05%. The inter-subject variation was also large. A sample size of 30 to 40 individuals would be necessary to demonstrate a 1 SD difference between individuals at a significance level of 0.05 and power of 0.80. It is concluded that the method is able to detect agents which stimulate monocytes at least equally well as FMLP, but may be impracticable for demonstrating changes in monocyte migration, or differences in the migratory response among different populations of monocytes.