Alveolar macrophages (AM) isolated from normal guinea-pigs and from those chronically exposed to endotoxin (LPS) were cultured in the presence of various concentrations of LPS (from 0.5 ng to 5 micrograms/ml). The presence of a neutrophil chemotactic factor (NCF) in culture supernatants was tested in migrations chambers. Contamination of all reagents has been tested using LAL test (Limulus Amoebocyte Lysate). The results indicate a production of NCF with low LPS concentrations (0.5 and 5 ng/ml) within the first 6 h of incubation: when larger doses are used the response decreases and a significant inhibition is observed with 5 micrograms LPS/ml (P less than or equal to 0.05). When contaminated medium was used, all responses observed were three times higher than with LPS-free medium (P less than or equal to 0.01). However, the response pattern was the same. AM from chronically exposed animals exhibit the same response patterns: the magnitude of NCF production was higher than with normal AM but not significantly. The data suggests that initial conditions of AM in vitro or in vivo with reference to LPS contamination have to be determined as they are of importance when AM NCF production has to be tested.