[en] Evidence for the increased immunogenicity of mastocytoma cells (P815) treated with 8-methoxypsoralen (8-MOP) and long-wavelength ultraviolet radiation (UVA) is presented. A highly tumorigenic clone (P1) became much less tumorigenic (tum^-1) after repetitive phototreatments with 8-MOP (16 ng/mL) and UVA (1 J/cm²). The yield of tum^- clones was proportional to the number of phototreatments. In a pilot study in which P1 cells were treated with three successive rounds of 8-MOP/UVA, one clone out of 73 was tum^-. In a second series of experiments, the P1 cells were treated 10 times and 4 out of 100 clones were much less tumorigenic. When some of the tum^- clones were administered intraperitoneally to DBA/2 mice, significant protection against challenge with the original P1 clone was observed. (Author)