Many research programs aimed to search anti-HIV agents for developing countries from local natural products, especially of botanical origin, which can play a role in the management of HIV-1 infection and AIDS. Recent antiviral screenings have demonstrated that some Euphorbiaceae species are effective against virus infections.[1,2,3] With the aim to find new antiviral compounds from the family, Euphorbia hirta (syn. E. pilulifera) was studied. This plant is used in the traditional medicine in many parts of Africa and Asia for the treatment of gastrointestinal disorders (diarrhoea, dysentery) bronchial and respiratory diseases (asthma, bronchitis) and ocular diseases (conjunctivitis). The present paper reports the antiretroviral activity of the extracts of E. hirta, which were investigated in vitro on MT4 human T lymphocyte cell line. The cytotoxicity of the extracts in uninfected cells was determined by MTT cell proliferation assay. First the direct effect of the aqueous extract on SIVmac251, HIV-1 and HIV-2 reverse transcriptase was determined. A dose-dependent inhibition of viral replication was observed in all three assays. Thereafter the HIV-1 inhibitory activity of aqueous (A) and 50% methanolic extracts (B) were compared, and found that the alcoholic extract showed higher antiretroviral effect (ID50=A: 9µg/ml, B: 5µg/ml). The 50% methanolic extract was then subjected to liquid-liquid partition with cyclohexane, dichloromethane and ethyl-acetate. Only the aqueous phase, remaining after extraction with organic solvents, exhibited significant antiviral activity, the lipophilic extracts revealed to be inactive. After removal the tannins from the aqueous extract, the viral replication inhibitory effect was remarkably decreased (ID50=81µg/ml), therefore it was concluded that most probably tannins are responsible for the high antiretroviral effect.