A detailed comparative assessment was made of the use of solid-phase-coupled antibodies in radioimmunoassay, by using an assay for human placental lactogen as a model system. The major advantages of the solid-phase technique are: (1) in common with the use of a second antibody, it is universally applicable; (2) separation can be carried out rapidly; (3) in contrast with some other techniques, the separation of antibody-bound and free antigen is virtually complete. The disadvantages when compared with other procedures are: (1) a considerable proportion of the antibody may be lost during the initial coupling reaction; (2) the tubes must be continuously mixed during incubation, and much effort is expended in removing and replacing the caps; (3) there is a decrease in the apparent affinity constant of the antibody after coupling, which is reflected in a lower sensitivity of the assay system. It is concluded that solid-phase antibodies are of greatest value in those systems in which the supply of antiserum is abundant, and in which the achievement of high sensitivity is not a requirement.