A solid phase antibody assay by means of alkaline phosphatase conjugated to antiimmunoglobulin is described. Specially designed microcuvettes were sensitized with influenza A antigen, and antibodies bound to it were assayed by anti-IgG alkaline phosphatase conjugate in a semiautomated photometer equipped with a programmable calculator. The sensitivity was found to be 200 times higher than HI- or CF-techniques, and the interassay variation was so small that twofold changes in antobody activity could be regarded as significant. Results from vaccinees indicated that serum samples could be collected at intervals of three to six days only to reach a serological diagnosis in clinical patients.