The chemotactic activity of serum has been monitored in eight patients with bronchial asthma after inhalation of an allergen. Lactoferrin and lysozyme were measured in serum simultaneously as markers of neutrophil and monocyte/macrophage activity and turnover. The heat‐labile chemotactic activity of serum showed a two‐phase response with an initial reduction in activity (median = 37.5 min after challenge) (P<0.05) followed by an increased activity (median = 60 min after challenge) (P<0.01). The consistency of this pattern was highly significant (P<0.0005). Heat‐stable chemotactic activity was significantly increased (P<0.01) 15 min after challenge.The initial reduction in heat‐labile chemotactic activity was observed only in those patients who developed a late asthmatic response and could suggest the involvement of immune complex‐mediated reactions since a similar reduction in heat‐labile chemotactic activity is produced byin vitroactivation of serum with aggregated IgG.The increased heat‐labile chemotactic activity showed a close relationship (P<0.01) to the extent of the immediate reduction in PEF‐rate. No relationship was discernible between the chemotactic activity and the eventually occurring rise in PMN blood count. In contrast the activity and turnover of PMNs after challenge as reflected by serum‐lactoferrin levels were significantly reduced (P<0.01). A close correlation (P<0.001) was found between the heat‐labile chemotactic activity in serum and the serum‐levels of lysozyme, suggesting the involvement of macrophages/monocytes in the asthmatic reaction and in the generation of the heat‐labile chemotactic activity found in serum aft