Previous results from our laboratory showed that in vitro 4-hydroxyoctenal, 4-hydroxynonenal (HNE) and 4-hydroxytetradecenal elicit a neutrophil chemotactic response in the presence of albumin. Data presented here indicate that other hydroxyalkenals (4-hydroxyundecenal and 4-hydroxy-pentadecenal) also exert a chemotactic attraction on rat pleural neutrophils. The different aldehydes tested show a wide activity-concentration range, the most powerful being 4-hydroxyoctenal. Unlike most chemoattractants, the hydroxyalkenals seem deprived of chemokinetic properties. The mechanism by which these compounds may act on granulocytes is analysed. Experiments investigating the deactivation phenomenon induced by HNE indicate that this aldehyde acts as a chemoattractant by triggering mechanisms different from N-formylmethionyl-leucyl-phenylalanine (fMLP). In fact, cells pre-incubated with this peptide fail to respond chemotactically to the same compound, but are still able to respond to HNE. Similarly, the preventive contact of neutrophils with HNE is unable to influence the chemotactic effect of fMLP.