Serodiagnostic tests for detecting green turtle (Chelonia mydas) antibody responses were developed to test the strength of association between exposure to spirorchid trematode antigens or herpesvirus antigens and having green turtle fibropapillomatosis (GTFP). Plasma samples from 46 captive-reared green turtles, including paired pre- and 1-yr post-inoculation samples from 12 turtles with experimentally induced GTFP, were found by enzyme-linked immunosorbent assay (ELISA) to be negative for antibodies to adult spirorchid (Learedius learedi) antigens. In contrast, all 12 turtles that developed experimentally induced GTFP converted within 1 yr from having negative to positive antibody reactivity to GTFP-associated herpesvirus antigens, whereas the three controls and four turtles that failed to develop tumors remained negative. Plasma samples from 104 free-ranging green turtles from two Florida (USA) coastal feeding grounds with different GTFP prevalences were tested by ELISA for antibodies to L. learedi adult antigens; and there was no statistically significant association between antibody prevalence and sampling site. When a low optical density cutoff value (0.15) was used to interpret ELISA results, 98% of the turtles from each site were spirorchid antibody-positive and there was no association between antibody reactivity to spirorchids and GTFP status. When a higher negative cutoff value was used, however, a statistically significant association between antibody reactivity to spirorchids and GTFP-free status was found. These results suggest that spirorchids do not have a role in GTFP pathogenesis. All 20 of the tumor-bearing lagoon turtles had antibodies to herpesvirus antigens whereas only two (10%) of the tumor-free reef turtles had detectable anti-herpesvirus reactivity. The strong association between antibody reactivity to herpesvirus antigens and GTFP status in both captive-reared and free-ranging turtles is consistent with the hypothesis that the transmissible agent that causes GTFP is a herpesvirus.