In order to test the biotechnological potential of the cellular slime mould Dictyostelium discoideum the cDNA coding for human antithrombin III was expressed in this microorganism. The 1392-bp antithrombin III cDNA was fused to the N-terminal coding part of the D. discoideum actin 6 gene. In constructs carrying this artificial N-terminal coding region only low amounts of antithrombin III were detected. However, constructs from which all actin coding nucleotides were removed produced significant amounts of anti-thrombin III, most of which was secreted into the culture broth. Stationary cultures (1.5 x 10(7) cells/ml) of certain stable transformants accumulated up to 1.0 microgram antithrombin III/ml culture medium within 24 h. The recombinant protein has a slightly smaller molecular weight in sodium dodecyl sulphate-polyacrylamide gels than authentic plasma antithrombin III and it is glycosylated, as determined by concanavalin A labelling.