Disclosed is a reagent for introducing a protein or gene into a cell. The reagent comprises a composition comprising a cationic amino acid-type lipid represented by the formula (I)-1 or the like. (I)-1 wherein L represents a single bond, -CONH- or -S-S-; M<SUP>1</SUP> represents -(CH<SUB>2</SUB>)<SUB>k</SUB>- or -(CH<SUB>2</SUB>CH<SUB>2</SUB>O)<SUB>k</SUB>- (where k represents an integer of 0 to 14); and m1 and m2 independently represent an integer of 11 to 21 (provided that, in the case of a reagent for introducing a gene into a cell, both of m1 and m2 are not an integer of 15).
