Throughout the 1970s, controversy centered both on immunoassay'sensitivity'per se and on the relative sensitivities of labelled antibody and labelled analyte methods. Our own theoretical studies in this period revealed that radioimmunoassay (RIA) sensitivities could be surpassed only by the use of very high specific activity non-isotopic labels in'non-competitive'designs, preferably based on the use of monoclonal antibodies. The time-resolved fluorescence methodology known as Delfia-developed in collaboration with the instrument manufacturer LKB/Wallac-represented the first commercial'ultra-sensitive'non-isotopic technique based on these theoretical insights, the same concepts being subsequently adopted in comparable methodologies relying on the use of chemiluminescent and enzyme labels. However, a second advantage of high specific activity labels is that they permit the development of'multi-analyte'immunoassay systems combining ultra-sensitivity with the simultaneous measurement of tens, hundreds or thousands of analytes in a small biological sample. This possibility relies on simple, albeit hitherto unexploited, physicochemical concepts. The first is that all immunoassays rely on measurement of Ab occupancy by analyte. The second is that, provided the Ab concentration used is' vanishingly small', fractional Ab occupancy is independent of both Ab concentration and sample volume. This leads to the notion of'ratiometric'immunoassay, involving measurement of the ratio of signals (eg fluorescent signals) emitted by two labelled Ab's, the first ('sensor'Ab) deposited as a microspot on a solid support, the second a'developing'Ab directed against either occupied or unoccupied sensor Ab binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)