Several theories of early sensory processing suggest that it whitens sensory stimuli. Here, we test three key predictions of the whitening theory using recordings from 152 ganglion cells in salamander retina responding to natural movies. We confirm the previous finding that firing rates of ganglion cells are less correlated compared to the pixels in naturalistic stimulus, although significant correlations remain. We show that while the power spectrum of ganglion cells decays less steeply than that of natural scenes, it is not completely flattened. Finally, retinal ganglion cells reduce the dimensionality of the photoreceptor signal, as evidenced by their significantly lower numbers. Whitening theory predicts that only the top principal components of the visual stimulus should be transmitted. By comparing the right singular vectors of stimulus and firing rate covariance matrices, we find evidence that supports this prediction.