The identification of nearly 3,500 cases of chikungunya virus (CHIKV) infection in U.S. residents returning in 2014 and 2015 from areas in which it is endemic has raised concerns within the transplant community that, should recently infected individuals become organ and/or tissue donors, CHIKV would be transmitted to transplant recipients. Thus, tests designed to detect recent CHIKV infection among U.S. organ and tissue donors may become necessary in the future. Accordingly, we evaluated 2 enzymelinked immunosorbent assays (ELISAs) for CHIKV IgM readily available in the United States using 1,000 deidentified serum or plasma specimens collected from donors between November 2014 and March 2015. The Euroimmun indirect ELISA identified 38 reactive specimens; however, all 38 were negative for CHIKV IgG and IgM in immunofluorescence assays (IFAs) conducted at a reference laboratory and, thus, were falsely reactive in the Euroimmun CHIKV IgM assay. The InBios IgM-capture ELISA identified 26 reactive samples, and one was still reactive (index > 1.00) when retested using the InBios kit with a background subtraction modification to identify false reactivity. This reactive specimen was CHIKV IgM negative but IgG positive by IFAs at two reference laboratories; plaque reduction neutralization testing (PRNT) demonstrated CHIKV-specific reactivity. The IgG and PRNT findings strongly suggest that the InBios CHIKV IgM-reactive result represents true reactivity, even though the IgM IFA result was negative. If testing organ/tissue donors for CHIKV IgM becomes necessary, the limitations of the currently available CHIKV IgM ELISAs and options for their optimization must be understood to avoid organ/tissue wastage due to falsely reactive results. C hikungunya virus (CHIKV) is an alphavirus transmitted from person to person via mosquito bites (1). Symptoms include fever, rash, and debilitating arthralgia; 15% to 60% of patients develop chronic arthralgia leading to arthritic joint damage (2). After a large CHIKV outbreak in India and southeast Asia in 2004 through 2006, in which nearly 2 million people became infected (3, 4), epidemiologists predicted that CHIKV might move to other geographic areas where the mosquito vectors are found (5). This prediction was realized in December 2013, when local transmission of CHIKV was reported on the Caribbean island of St. Martin (6). CHIKV infection has since spread throughout the Caribbean basin (7) and is now also endemic in Mexico, Central America, and South America and in the Caribbean island nations. In conjunction with this outbreak, 3,490 cases in U.S. residents (from 49 of 50 states) were reported to the CDC during 2014 and 2015; 3,478 cases represented infections acquired during international travel to areas where CHIKV is endemic, whereas 12 cases represented local transmission (8).There is concern within the transplant community that CHIKV could be transmitted from organ and/or tissue donors to recipients. Donor-derived transmission of other mosquito-borne viruses with similar epidemi...