A polyamino acid oligonucleotide carrier comprising a poly(lysine-serine) random copolymer. The carrier combines with an oligonucleotide without causing precipitation and releases the combined oligonucleotide smoothly in a cell. Hence it is useful as a carrier for in vivo transportation of a drug for oligonucleotide gene therapy against various types of genopathy and viral diseases such as AIDS. Further it contributes greatly to the creation of various useful animals and plants, such as virus-resistant plants, by genetic engineering techniques.