Realistic models based on the renormalizable grand unified theories have varieties of scalars, many of which are capable of mediating baryon and lepton number non-conserving processes. We identify all such scalar fields residing in 10, 126 and 120 dimensional irreducible representations of SO(10) which can induce baryon and lepton number violating interactions at tree level. Explicitly computing their couplings with the standard model fermions, we derive effective operators including the possibility of mixing between the scalars stemming from a given representation. We find that such interactions are mediated by only three sets of scalars: T (3, 1, −1/3), T (3, 1, −4/3) and T(3, 3, −1/3) and their conjugates. In the models with 10 and 126, only the first has appropriate couplings to mediate the proton decay. While T and T can induce baryon number violating