The anti-inflammatory influence of dapsone may involve suppression of neutrophil chemotaxis to selected attractants, but other actions of the drug are likely also involved. We have discovered that dapsone may suppress migration of neutrophils to extravascular sites through inhibition of adherence functions required for neutrophil recruitment. Neutrophil adherence mediated by integrins (CD11/CD18 or Mac-1 family receptors) was measured in vitro in terms of binding of stimulated cells to albumin-coated wells of microtiter plates, using phorbol myristate acetate (PMA) and N-formylmethionyl-leucyl-phenylalanine (FMLP) as stimuli. Adherence was assessed by staining attached cells with crystal violet dye and measuring the dye concentration at OD59a using an automated plate reader. The role of integrins in this assay was confirmed by the ability of anti-integrin antibody to suppress stimulated neutrophil adherence. The OD590 value for cells adhering to albumin in the absence of stimulus and dapsone averaged 0.2 ± 0.04 (SEM) over five experiments. In the presence of 0.1 μM PMA or 10-6 M FMLP, the OD590 values averaged 0.88 ± 0.1 and 0.75 ± 0.12, respectively. Dapsone did not affect unstimulated neutrophil adherence but, when present with stimulus, produced a dose-related inhibitory effect on adherence. Fifty, percent inhibitory doses were approximately 150 μg/ml dapsone for both stimuli. Sulfapyridine reproduced the inhibitory effect of dapsone, but two structurally related compounds, hydrochlorothiazide and furosamide, did not. The observed ability of dapsone to inhibit neutrophil chemotaxis under agarose to FMLP and interleukin-8 may also be explained by interference with integrin-mediated adherence required for motility in this assay system. To consider if dapsone might have a similar inhibitory influence on neutrophil adherence in vivo, we tested the stimulated adherence function of neutrophils isolated from three individuals on dapsone therapy for dermatitis herpetiformis. Stimulated adherence of patients' cells averaged less than 40 percent of the control value. Suppression of leukocyte integrin function may therefore also contribute to the ability of dapsone to inhibit neutrophil infiltration in neutrophilic dermatoses.