Immunoassays for the detection of viral antigens in clinical specimens and virus-specific IgM responses in serum have shortened the time required to make a laboratory diagnosis of several infections. A range of antigen detection systems are available, varying in sensitivity, complexity, and expense, and each may have a role to play depending upon the laboratory setting. Technical advancements to eliminate false-positive results in solid-phase IgM assays have provided an awareness of very early IgM responses in diseases such as rubella, hepatitis A, and mumps. When clinical specimens contain large numbers of virus particles, a rapid diagnosis is easily made using electron microscopy. Detection of antigens, virus particles, and IgM responses is creating increased demands for viral diagnostic services in primary care settings. Other approaches using sensitive probes for viral nucleic acids or enzymes will also serve as viable laboratory techniques in the future.