The interaction of alpha 1-Antitrypsin, the major serine protease inhibitor in plasma, with pathogenic Trichomonas vaginalis and the acquisition by trichomonads of this host protein from normal human plasma were investigated. alpha 1-Antitrypsin acquired by intact parasites could not be removed by repeated washings in phosphate-buffered saline. Saturation kinetics were observed after incubation of glutaraldehyde-fixed organisms with 125I-labeled alpha 1-antitrypsin. Evidence suggesting that specific parasite membrane sites were responsible for trichomonal acquisition of alpha 1-antitrypsin was obtained through competitive binding experiments using purified preparations of homologous versus heterologous plasma proteins. No evidence of degradation of bound antitrypsin by live parasites was observed. The avid binding of alpha 1-antitrypsin by pathogenic T. vaginalis after incubation in normal human plasma was demonstrated by using sensitive electrophoretic and immunodetection techniques. Radioimmunoprecipitation of intrinsically labeled, detergent-solubilized extracts of T. vaginalis incubated with monospecific antisera against alpha 1-antitrypsin and other human plasma proteins revealed the inability of parasites to biosynthesize any substance cross-reactive with host plasma proteins. Finally, T. vaginalis organisms pretreated with alpha 1-antitrypsin inhibited trypsin caseinase activity in an in vitro assay. The implications of these observations are discussed.