We present longitudinal magnetic field measurements (Bz) for the Herbig Ae primary and the T Tauri secondary in the SB2 system HD104237. These measurements were carried out using high spectral resolution observations obtained with the High Accuracy Radial velocity Planet Searcher in polarimetric mode, installed at the ESO La Silla 3.6m telescope. In agreement with previous studies of Herbig Ae stars, the longitudinal magnetic field in the primary is rather weak, ranging from 47 to 72 G. The secondary component possesses a variable, much stronger magnetic field, up to 600 G, as expected for a magnetically active T Tauri star. We estimated the rotation period of the primary, P_rot = 4.33717 ± 0.00316 d (=104 ± 0.08 h), from metal line equivalent width variations.