We present a new model to predict chemical shifts for biological molecules. It is simple, fast, and involves a limited number of parameters. It is particularly adapted to be used in molecular dynamics studies with a molecular mechanic potential. We test the model for polyamines, which are rather small molecules, as well as for proteins for which a lot of NMR chemical shifts are available. The tests show that our simple and fast model is competitive, by its accuracy, with sophisticated models specifically developed for proteins. It was also seen to be successful for polyamines.