As an attempt to determine levels of benefit in a marketing relationship a model rooted in the biological sciences has been used. The model uses degrees of symbiosis in parasitology to investigate the complicated relationship structure between financial institution, charity and cardholder, found in the case of the affinity credit card. Taking one focal point of this triadic relationship, interviews were conducted with the relationship managers from a sample of UK charities. The results of the qualitative research suggest that where both parties in the relationship were proactive and wanted the relationship to flourish it was perceived to be mutually beneficial. The financial institution is in a win-win situation by gaining access to the charity database, but the charity has to demonstrate matching enthusiasm otherwise the affinity card will not reach its potential for the charity. The research also suggests that the perceptions of the mutuality of the relationships do not always match up to the reality. If, however, both organisations fully understand the implications of the possible degrees of symbiosis within the relationship, then mutuality is possible and the model developed here should help that understanding.