Immunoaffinity methods are presently finding limited applications in e
nvironmental analysis because many target compounds are best handled i
n non-polar solvents such as hexane. These solvents can be detrimental
to protein molecules, particularly when the solvating water which is
necessary for bioactivity is lost. Consequently, a generic immunoassay
principle which would allow the provision of samples dissolved in hex
ane was sought. A competitive enzyme-linked immunosorbent assay (ELISA
) method for the measurement of aqueous parathion was developed, optim
ized and then adapted to the measurement of the analyte without its pr
ior removal from hexane. The parathion-specific antiserum was encapsul
ated in reverse micelles composed of Aerosol OT and having aqueous cen
tres, and a solution of parathion in hexane was then allowed to compet
e for antibody binding sites with the antigen-coated surface of a micr
otitre plate. The calibration graph for the hexane-compatible ELISA me
thod is displaced from its aqueous counterpart in correspondence with
the partition coefficient of parathion between water and hexane, this
method being about 10(4)-fold more sensitive. It is probable that the
thermodynamic barrier to the transfer of parathion from an organic bul
k phase to the aqueous interior of a reverse micelle balances the anti
gen-antibody binding strength and thus reduces the potency of parathio
n as an antigenic competitor. Means are suggested for tuning the calib
ration graph of the hexane-compatible ELISA method to the analyte conc
entration range that is of interest.