A quantitative FT-IR spectroscopic method has been developed for the trace analysis in chlorinated organic solvents of transition-metal carbonyl-labeled bioligands. In order to illustrate the widespread analytical potential of the method, three derivatives of the female hormonal steroid 17 beta-estradiol, containing Cr(CO)3, Cp2Mo2(CO)4 (Cp = eta 5-C5H5), and Co2(CO)6 as labels, and the anticonvulsant drug phenobarbital, labeled with (eta 5-C5H4)Mn(CO)3, were examined. The cobalt carbonyl marker proved to be the best sulted for quantitative analysis purposes, and the minimum tracer quantity detectable for this particular marker (64 scans, 4-cm-1 resolution, 3.5 min) was optimized in CCl4 solution at about 300 fmol (or 0.3 pmol, 180 pg) by using an ultralow volume (23.0 microL), gold light-pipe IR solution cell and a liquid nitrogen cooled, InSb (indium antimonide) IR detector. The repeatability of this radically different analytical procedure over the concentration range 1.0 x 10(-6) to 5.0 x 10(-8) M was good (coefficient of variance less than or equal to 6%) and the method provides the basis for a new immunological test--carbonylmetalloimmunoassay (CMIA).