Mesosphaerum pectinatum (L.) Poit (Laminaceae), popularly known in Brazil as “sambacaitá”, is an herbaceous plant extensively used in Brazilian Northeast and southeastern region of Mexico for the treatment of various illness. In Sergipe (Brazil), it is used for treating inflammations, bacterial infections, pain and cancer. Although some studies prove its anti-inflammatory and antinociceptives effects, there is little information about anti-cancer properties. The aim of the study was to investigate the antiproliferative effect of extracts from M. pectinatum on MDA-MB-231 breast cancer tumor line. The ethanol extracts from leaves (MPL), inflorescences (MPFl), branches (MPB) and roots (MPR) of M. pectinatum obtained by turboextraction were tested against breast cancer tumor line. The cellular viability was determined by MTT colorimetric assay, and is presented as IC50 (concentration required to inhibit the cell proliferation by 50%). The results showed that MPL and MPFl presented the best antiproliferative effects against this tumor cell line. These extracts (100 ug/mL) were significantly effective in inhibiting cell proliferation after 48 h of incubation. Then, the IC50 values these extracts was calculated. MPF and MPFl presented IC50 of 45.91 ug/mL and 52.01 ug/mL, respectively. 1H NMR analysis was performed in order to identify the major components of the extracts tested. After thorough analysis, it was observed that in both extracts (HPL and HPFl), the same major secondary metabolites were present, whereas were absent in MPB and MPR. Thus, these compounds may be responsible for the antiproliferative activities of the tumor cells tested. The 1H NMR signals present in the spectrum are characteristic of the class of compounds named pectinolides, metabolites already identified in M. pectinatum. In addition to other signals, the presence of four hydrogen signals from the acetyl groups at the 1H NMR spectrum suggests that the major pectinolide present in the most active extracts is pectinolide “E”. Other pectinolides have already been tested against several tumor cell lines, however pectinolideo "E" has not yet. After the purification of this pectinolide, the assays against tumor cells will be performed again to confirm the antiproliferative effects of this substance.