We observed the field of GRB 100728B (Swift trigger 430172; Morris et al., GCN 11009) simultaneously in JHK with GROND (Greiner et al. 2008, PASP 120, 405) mounted at the 2.2 m MPI/ESO telescope at La Silla Observatory (Chile). Observations started at 10:34 UT, 2 minutes after the GRB trigger. They were performed during astronomical twilight at an average airmass of 1.2. We detect a single point source at position reported by Perley et al. (GCN 11007), Morris et al. (GCN 11009), Ivarsen et al. (GCN 11011) and Elenin et al. (GCN 11012). Based on the first 4 min of total exposures in JHK, we estimate preliminary magnitudes (all in AB system) of J = 16.4 +- 0.1 mag, H = 16.2 +- 0.1 mag and K = 16.0 +- 0.1 mag Given magnitudes are calibrated against 2MASS field stars and are not corrected for the expected Galactic foreground extinction corresponding to a reddening of E_(B-V)=0.07 mag in the direction of the burst (Schlegel et al. 1998).