When will bystanders of workplace mobbing show antisocial or prosocial behaviour toward the victim? Results of a 2 × 2 vignette study (N = 177) suggest that high perceived responsibility of the victim for the onset of the mobbing evokes anger and consequently antisocial bystander behaviour, whereas low perceived responsibility generates sympathy and consequently prosocial bystander behaviour. The results further indicate that bystanders will show more antisocial behaviour and less prosocial behaviour toward the victim when they anticipate stigma by association. The implications of these results for interventions seeking to influence bystanders' behaviour in the context of workplace mobbing and for further research on this bystander behaviour are discussed. Copyright © 2015 John Wiley & Sons, Ltd.