A study of the magnetic properties of monocrystalline YCo 4 B samples is reported. A novel property, i.e. a first-order magnetisation process (FOMP), has been found in a wide temperature range. Besides this FOMP, 'FOMP-like' processes have been observed at certain temperatures. Apart from an anomaly corresponding to a spin-reorientation transition at 155 K, the temperature dependences of the magnetisation and the AC susceptibility show other anomalies at 30 and 80 K. The magnetocrystalline anisotropy of YCo 4 B is discussed in terms of a competition between contributions arising from the Co ions at different crystallographic sites.