Abstract: Novel cellulosic sizing agents are described. They are ring-opened aziridinyl homopolymers of ##EQU1## wherein R.sub.1 and R.sub.2 are hydrogen or lower alkyl; R.sub.3 is hydrogen or methyl; R.sub.4 is an organic hydrophobic group (e.g., C.sub.8 -C.sub.24 alkyl radical); and X is oxygen or amido nitrogen. The above homopolymers can also be blended with cationic starch to form useful sizing compositions. These sizing agents are normally employed as aqueous emulsions. E.g., paper having excellent sizing properties was prepared from a paper pulp containing a mixture of cationic starch and poly[stearyl 3-(1-aziridinyl)propionate].