The intraneutrophilic concentrations of lactoferrin, myeloperoxidase, collagenase and chymotrypsin-like cationic proteins were measured sequentially during acute bacterial infection. The serum levels of lactoferrin and myeloperoxidase were also followed as well as the 'eosinophil' cationic protein as a marker for eosinophil leucocytes. During the early course of infection there was a profound but reversible decrease of intraneutrophilic lactoferrin. The levels of cellular collagenase and chymotrypsin-like cationic proteins also tended to decrease reversibly during day 2-8 in most cases; myeloperoxidase levels were normal except for two cases. Serum myeloperoxidase and lactoferrin correlated with blood neutrophil counts. In spite of the absence of peripheral eosinophils the 'eosinophil' cationic proteins of serum were increased on the first day of infection, which may reflect increased eosinophil turnover.