en Other Other Original Article Original Article <span style="font-family:Tahoma;"><span style="font-size:12px;"><span style="unicode-bidi:embed"><span style="line-height:107%">This study observed <em>in vitro</em> screening, purification and identification of cholinesterase inhibitors from the microalgae <em>Phormidium retzii</em>. Mixed microalgal culture was screened from freshwater samples for Phormidium sp. Single colony was purified and authenticated as <em>P. retzii</em>. Acetylcholinesterase (AChE) enzyme was purified from hRBC ghost. Sequential extraction of <em>P. retzii</em> was performed using organic solvents. Cholinesterase enzyme activity and its inhibition by various extracts were then tested. The active fractions were then subjected to partial purification and characterization. Petroleum ether extract of <em>P. retzii</em> showed maximum inhibition of 68.6 % against AChE while other solvent extracts showed no inhibition. Seven fractions were obtained from the active extract using thin layer chromatography. Among which fraction no. 5 showed maximum inhibition of 86.37 % towards AChE. Fraction no. 5 when subjected to GC-MS led to determination of the active principle as stigmasterol. The maximum inhibition of stigmasterol (0.45&micro;M) was 81.2&plusmn;0.08% with IC50 value of 0.214. Stigmasterol from <em>P. retzii</em> inhibited AChE projecting itself as safer drug for Alzheimer&rsquo;s disease with minimal side effects<span dir="RTL" lang="FA">.</span></span></span></span></span> Acetylcholinesterase, Alzheimer’s disease, inhibition, microalgae, Phormidium retzii, stigmasterol 100 107 http://ijmcmed.org/browse.php?a_code=A-10-6668-1&slc_lang=en&sid=1 Rincy Yesudas drrincyyesudas@gmail.com 100319475328460027838 100319475328460027838 Yes CMS College of Science and Commerce, Chinnavedampatti (PO), Coimbatore, Tamil Nadu, India. Vinoth Kumar Thirumalairaj dr.tvinothkumar@gmail.com 100319475328460027839 100319475328460027839 No CMS College of Science and Commerce, Chinnavedampatti (PO), Coimbatore, Tamil Nadu, India. Geetharamani Durairaj geethavinoth@gmail.com 100319475328460027840 100319475328460027840 No Dr. N.G.P. Arts and Science College, Coimbatore, Tamil Nadu, India. Amrutha Chacko amrutha027@gmail.com 100319475328460027841 100319475328460027841 No CMS College of Science and Commerce, Chinnavedampatti (PO), Coimbatore, Tamil Nadu, India. Lakshmanasenthil Shanmugaasokan lakshmanasenthil@gmail.com 100319475328460027842 100319475328460027842 No CAS in Marine Biology, Annamalai University, Chidambaram, Tamil Nadu, India. Suja Gunasekaran sujaooty@gmail.com 100319475328460027843 100319475328460027843 No CMS College of Science and Commerce, Chinnavedampatti (PO), Coimbatore, Tamil Nadu, India.