Three experiments employed a metacontrast masking procedure to examine the extent and nature of priming effects from visual stimuli not consciously perceived. The results showed effects of unconscious stimuli on subsequent target responses that (1) were more consistent, reliable, and not subject to strategic control, as compared with consciously perceived stimuli (Experiment 1); (2) produced both facilitation and interference of subsequent processing (Experiment 2); and (3) did not influence indirect response-related levels of processing (Experiment 3). These results demonstrate that color and form attributes of unconscious stimuli are sufficiently registered within the visual system to influence behavior, and that some of these unconscious effects occur at early levels of stimulus encoding, prior to higher level perceptual and response-related processes.