Neutrophil chemotactic activity was detected in the fluid of Toxoplasma gondii cultures by the agarose plate and the Boyden chamber methods. Toxoplasma culture fluid was obtained by incubating the tachyzoites at 37 °C in a 5% CO2 atmosphere for 6 h in Dulbecco's modified Eagle's minimum essential medium containing 10% heat-inactivated foetal calf serum. Soluble extracts from tachyzoites had negligible activity, indicating that the chemotactic factors were metabolites secreted from tachyzoites. The chemotactic activity in Toxoplasma culture fluid depended on the number of tachyzoites and the period of incubation. The substances responsible for neutrophil chemotaxis were two heat-labile peptides with estimated Mr 4·5 and 14 kDa with N-terminal groups free.
