Abstract  In 1990 Rivest introduced the cryptographic hash function MD4. The compress function of MD4 has three rounds. After partial attacks against MD4 were found, the stronger mode RIPEMD was designed as a European proposal in 1992 (RACE project). Its compress function consists of two parallel lines of modified versions of MD4-compress. RIPEMD is currently being considered to become an international standard (ISO/IEC Draft 10118-3). However, in this paper an attack against RIPEMD is described, which leads to comparable results with the previously known attacks against MD4: The reduced versions of RIPEMD, where the first or the last round of the compress function is omitted, are not collision-free. Moreover, it turns out that the methods developed in this note can be applied to find collisions for the full MD4.