Abstract: Objective To explore the effect of usnic acid on Toxoplasma gondii tachyzoites in vitro. Methods There are four groups named as （＋）-usnic acid group, acetylespiramycin group, DMSO group and normal saline group. Groups of （＋）-usnic acid and acetylespiramycin were further divided into 4 subgroups with final concentration of 5, 10, 25, 50 μg/ml respectively. Normal saline group and DMSO group were respectively given equal volume normal saline and 1% DMSO. Each group have 15 parallel tubes with 1 ml （1×10⁶/ml） T. gondii tachyzoites aqueous suspension. At 1 h, 2 h and 4 h after drug treatment, tachyzoites were counted by light microscope with 0.4% Trypan blue staining. Tachyzoites in aqueous suspension was collected, and washed 3 times by PBS solution. Normal mice were inoculated intraperitoneally and observed for three generations. The cultivated rat cardiofibroblasts were then infected in vitro with T. gondii tachyzoites. At the same time, rat cardiomycytes invasion by T. gondii tachyzoites was investigated. Results At 4 h treated by 10, 25 and 50 μg/ml （＋）-usnic acid, 100% T. gondii tachyzoites were stained. Some tachyzoites were swelling, blunt or round in the two ends; and granules appeared in the cytoplasm, the nuclei were deep stained. The changes of tachyzoites in acetylespiramycin group were similar to （＋）-usnic acid group, 100% T. gondii tachyzoites were stained in 50 μg/ml acetylespiramycin subgroup. In inoculation tests, mice died at 8th to 9th days in 5 μg/ml （＋）-usnic acid subgroup and numerous tachyzoites were detected in ascites. However, most mice survived to be killed in the other （＋）-usnic acid subgroups and the tachyzoites were not found in ascites. All mice in acetylespirmycin groups died at 6th to 8th days after inoculation and many tachyzoites or psudocysts were observed in mice ascites. In infecting cell tests, the cultivated rat cardiofibroblasts were infected in vitro by the tachyzoites after treated with 5 μg/ml （＋）-usnic acid for 4 h， and psudocysts were formed in infected cells. It was negative in the other subgroups of （＋）-usnic acid. But the cultivated rat cardiofibroblasts were infected to varying degree in acetylespiramycin groups, normal saline group and DMSO group. Conclusion （＋）-Usnic acid has a remarkable effect on T. gondii tachyzoites.

Key words: Usnic acid, Acetylespiramycin, Toxoplasm gondii, Tachyzoite