A series of semi-purified diets containing 20% fat by weight, of increasing proportions (0, 5%, 10%, 15% or 20%) of polyunsaturated sunflower oil mixed with hydrogenated saturated cottonseed oil, was fed to groups of Skh:HR-1 hairless mice during induction and promotion of photocarcinogenesis. The photocarcinogenic response was of increasing severity as the polyunsaturated content of the mixed dietary fat was increased, whether measured as tumour incidence, tumour multiplicity, progression of benign tumours to squamous cell carcinoma, or reduced survival. At the termination of the study approximately 6 months following the completion of the 10-week chronic UV irradiation treatment, when most mice bore tumours, the contact hypersensitivity (CHS) reactions in those groups supporting the highest tumour leads (fed 15% or 20% polyunsaturated fat), were significantly suppressed in comparison with the mice bearing smaller tumour loads (fed 0, 5% or 10% polyunsaturated fat). When mice were exposed acutely to UV radiation (UVR), a diet of 20% saturated fat provided almost complete protection from the suppression of CHS, whereas feeding 20% polyunsaturated fat resulted in 57% suppression; the CHS of unirradiated mice was unaffected by the nature of the dietary fat. These results suggest that the enhancement of photocarcinogenesis by the dietary polyunsaturated fat component is mediated by an induced predisposition to persistent immunosuppression caused by the chronic UV irradiation, and supports the evidence for an immunological role in dietary fat modulation of photocarcinogenesis in mice.