This invention relates to retroviral regulatory proteins or fragments thereof, or interferon alpha protein or fragments thereof, which are carboxymethylated. This chemical modification leads to new proteins or fragments which are biologically inactive but preserve their immunogenicity (toxoids). These proteins or fragments thereof, or interferon alpha or fragments thereof, can be utilized in the treatment and prevention of retroviral infections. The invention also relates to a pharmaceutical composition comprising at least one carboxymethylated protein or fragment of the invention, together with a pharmaceutically acceptable carrier. The invention also relates to a vaccine comprising at least one of the carboxymethylated proteins or fragments of the invention, together with an immunologically acceptable carrier. The invention also relates to a process for obtaining an immunogenic yet not toxic retroviral regulatory protein or fragment, or interferon alpha or fragment. The invention also relates to a method of inducing an immune response in a mammal, comprising administering the vaccine of the invention to a mammal in an immunologically effective amount.