Background: Antioxidants can come from plants that contain flavonoids such as ironwood. The bark of ironwood is declared effective to be used as a mouthwash because of the content of flavonoids, phenolics, tannins, saponins, alkaloids, and terpenoids. Ironwood bark extract to be used as an alternative material must be safe for oral tissues, so it is necessary to do a toxicity test. Objective: To analyze the toxic effect of ironwood bark extract through IC50 on BHK-21 fibroblast cells. Methods: This study was a pure laboratory experimental study with posttest-only control group design, consisting of 12 groups with 10 treatment groups given ironwood bark extract at concentrations of 5%, 15%, 25%, 35%, 45%, 55%. , 65%, 75%, 85%, and 95% and 2 control groups, namely cell control and media control. It was repeated 3 times so that the total sample was 36 samples. Toxicity test media used MTT assay which produced color absorbance and cell viability was calculated. Results: The results showed that the cell viability of the entire treatment group was >60% so it had no toxic effect. In addition, based on the IC50> 0.1%, which is 3.746%, it has no toxic effect on BHK-21 fibroblast cells. Conclusion: Ironwood bark extract at concentrations of 5%, 15%, 25%, 35%, 45%, 55%, 65%, 75%, 85%, and 95% had no toxic effect on BHK-21 fibroblast cells.   Keywords: BHK-21 fibroblast cells, Ironwood bark extract, , toxicity ABSTRAKLatar Belakang: Antioksidan dapat berasal dari tanaman yang mengandung flavonoid seperti ulin. Kulit batang ulin dinyatakan efektif untuk dijadikan obat kumur karena adanya kandungan flavonoid, fenolik, tanin, saponin, alkaloid, dan terpenoid.  Ekstrak kulit batang ulin untuk dijadikan sebagai bahan alternatif harus bersifat aman untuk jaringan rongga mulut sehingga perlu dilakukan uji toksisitas. Tujuan: Menganalisis efek toksik ekstrak kulit batang ulin melalui IC50 terhadap sel fibroblas BHK-21. Metode: Penelitian eksperimental laboratoris murni dengan posttest-only with control group design, terdiri dari 12 kelompok dengan 10 kelompok perlakuan yang diberi ekstrak kulit batang ulin konsentrasi 5%, 15%, 25%, 35%, 45%, 55%, 65%, 75%, 85%, dan 95% dan 2 kelompok kontrol, yaitu kontrol sel dan kontrol media. Dilakukan 3 kali pengulangan sehingga total sampel berjumlah 36 sampel. Media uji toksisitas menggunakan MTT assay yang menghasilkan absorbansi warna dan dilakukan perhitungan viabilitas sel. Hasil: Viabilitas sel seluruh kelompok perlakuan adalah >60% sehingga tidak memiliki efek toksik. Selain itu, berdasarkan nilai IC50>0,1% yaitu sebesar 3,746% tidak memiliki efek toksik terhadap sel fibroblas BHK-21. Kesimpulan: Ekstrak kulit batang ulin pada konsentrasi 5%, 15%, 25%, 35%, 45%, 55%, 65%, 75%, 85%, dan 95% tidak memiliki efek toksik terhadap sel fibroblas BHK-21. Kata kunci: Esktrak kulit batang ulin, sel fibroblas BHK-21, toksisitas.