Singlet oxygen ((1)O(2)) produced during inflammatory reactions and during photodynamic therapy deactivates by producing in tissues secondary reactive oxygen species and peroxides (SOS) as well as other degradation products. We investigated the influence of animal species on SOS production secondary to standardized (1)O(2) production by performing in vitro experiments with rose bengal as the (1)O(2) producer, human serum (HS) as a control, sera derived from various animal species, and dichloro-dihydro-fluorescein as a nonspecific marker that becomes fluorescent when oxidized. The overall SOS production in HS from a presumed healthy cohort of 53 donors (31 males and 22 females) gave a mean "normal" value of 0.91 compared with a previous pool of 75 male sera samples. SOS production after a photo-reaction was two or four times lower in HS than in fetal calf serum or mice sera, respectively. In mice, the "nude" characteristic increased even more than in the SOS production. In the Aves order, this production appeared to be distributed randomly according to the number of branches after the appearance of Amniotas. For primates, SOS production appeared to decrease linearly with the number of branches (R(2) = 0.98). Adding hemolysates from complete bloods to the corresponding sera induced an increase in SOS production in all species, proportional to the production in sera. These findings should be kept in mind when interpreting results from studies of secondary reactive oxygen species-induced pathways following (1)O(2) production, regardless of its origin.