Various polymorphonuclear leukocyte (PMN) functions are dependent on an intact intracellular cytoskeleton consisting of the microtubules and the microfilaments. To investigate the microtublule system in PMNs we observed the spontaneous, Colchicine and Diamide induced cap-formation by fluorescence microscopy ion PMNs obtained from children with bacterial and viral infections demonstrated with 47 +/- 1% a significantly increased number of spontaneous capped PMNs compared to 22 +/- 1% capped cells obtained from controls. Furthermore, 52 +/- 2% PMNs of patients on immunosuppressive therapy exhibited spontaneous surface capping. There was no significant elevation in the number of capped PMNs (30 +/- 2%) obtained from children with viral infections. Colchicine and Diamide increased the number of capped cells in control PMNs as well as in PMNs from patients to 69 +/- 1% and 67 +/- 1%, respectively. Since the increased spontaneous cap formation in PMNs is associated with a defect of microtubule assembly, the various leukocyte function defects described in patients with bacterial infections, bronchial asthma or on immunosuppressive therapy may have to be considered the consequence of an altered microtubule system.