Only a limited number of detailed investigations have becn made on the cstcrases in the cilatcd prot.ozoa. Bayer and TVen~c ( '36) who identified choline, acetylcholine and a n esterasc in paramoecia f o n d that the hydrolysis of acetylcholinc was facilitatcd at low tcmpcraturcs. Wcnsc ( '38) maint.ains that b0t.h refrjgeratd and filtored extra& of paramoecia contain srnallcr numbcrs of bacteria and low concentrations of csterasc inhibitor. On the ot.her hand, inc,ixbatod extract,s and unfiltered cxtracts contain grcat,cr numbcrs of bac tcria, and high c,oncent.r a tions of e s t eras e inhibitor. TJiid o r the latter conditions hydrolysis of acetylcholinc was rctardctl. Bullock and Nachmansohn ( '42) were unable to ident.ify acct.ylcholi nc s t crasc i TI 7'0 r n na o e c i . w ~ ,in 1. 1 11 h i ( : r o n d e a tu na. Fcnncll ( '44) found that aqueous cxtracts of para.moecia intlixced a slow contraction of the rcctns abdorninis of the frog and that the degree of contraction wa,s enhanced by eserinc sulfate, although some extracts wcrc without cffcct. An cstcraso was identified in .Tetrah?y??ar!nsr gel& S which specifics1 Iy split acct,ylcholine but ayqmrcntly did not I.iycl~oIyzc Inutyrylcholine (Seaman and Houlihan, '51). E'cnncll and kfarzke ( '54) using 1 ThiR stutly was stippartcd by grants-in-aid from the SationaI Tnst.itutc of Arthritis anid Yetabolia Diseasrs (snuill grant, A-1491 ), Sntionnl Iristitutcs of IIealth, and 5fichigs.n Statc Univcrsiby, -411 University R.csetircih Grant. ( p o j w t 110. 11133).