Plants may have the ability to modulate immune responses. In the present study, the effects of three plants belonging to Labiatae family, each traditionally used for the treatments of infections and inflammatory diseases, as well as the role of thymol (as one the major components of these plants), were investigated for their potential to affect the activation of lymphocytes. Four organic extracts of Thymus vulgaris and two other plants (i.e., T. daenensis and Zataria multiflora) were prepared. The effect of the extracts on mitogen (PHA)-stimulated peripheral blood lymphocytes was determined using a cell proliferation assay. The hexane extracts obtained from the three plants showed the strongest inhibitory effects on PHA-induced proliferation. Use of preparative thin layer and gas chromatographies in conjunction with the proliferation assay confirmed that thymol was the major component responsible for the observed effects from the three plants. Thymol inhibited inducible lymphocyte proliferation in a concentration-dependent manner, with reductions ranging from 62.8% at 50 µg/ml to 89.8% at 200 µg/ml (> 0.1 µg/ml (p < 0.01). Flow cytometric analysis using propidium iodide staining showed that the inhibitory effect of thymol at 200 µg/ml was due to a cytotoxic activity. In conclusion, the three Labiatae plants studied here each showed immunosuppressive effects against lymphocytes and it was most likely that thymol was the compound in these plants responsible for this effect.