The role of the clinical microbiology laboratory in transplantation. The clinical microbiology laboratory plays a central role in any transplantation program. Pretransplantation microbiologic testing often determines prophylactic treatment, donor selection, and blood product use. With suspected infection, rapid microbiologic tests permit prompt therapy but are challenged by an ever-changing diversity of potential pathogens and limited specimen size. Antigen detection and the polymerase chain reaction with nucleic acid hybridization are newer methods that promise earlier detection of such serious infections as disseminated aspergillosis and primary cytomegalovirus and may reveal new microbial causes of various posttransplantation syndromes. 