Abstract: The southern galactic cluster NGC 4103 has been studied photometrically. For 512 stars brighter than V = 17 m magnitudes and colours are given. Spectral types are derived for 17 of the brightest using U, B, V photometry. Spectroscopic observation of star 12 reveals MK B2IVe. Probably no stars having types earlier than B2 exist in the cluster. The mean colour excess E B–V = 0.30. The apparent distance modulus was found to be 12.0. With an absorption of 1.0 the true distance modulus and the distance are 11.0 and 1600 parsecs respectively. An age of 27 × 10 6 years from the turn off point on the main sequence was found. The eclipsing variable AI Crucis is the 5th brightest star of the cluster and almost certainly a member. Popper and Thackeray observed line doubling in the spectrum (B2IV + . . . ) of this star.