The solid-phase radioimmunoassay procedures for serum digoxin and cortisol with the Concept-4 were evaluated for specificity, correlation with other radioimmunoassay procedures, parallelism, recovery, carryover, and precision. 1. The detection limits of the digoxin and cortisol assays were 10 pg/tube and 40 pg/tube, respectively. These detections correspond to serum concentrations of 0.25 micrograms/liter and 20 micrograms/liter, respectively, for digoxin and cortisol. 2. Specificity of the digoxin assay was characterized by a 18.3% cross reaction with digitoxin. The anti-cortisol with 11-deoxycortisol, cross reaction with 11-deoxycortisol. 3. The values obtained with the Concept-4 for both procedures demonstrated acceptable correlations with other existing RIA procedures. 4. Dilution studies conducted to determine the parallelism of the 2 assays indicated that both assays demonstrated excellent proportionality between the volume of specimen and the quantity of analyte measured. 5. Analytical recoveries of digoxin and cortisol added to the individual assays average 95% and 101%, respectively. 6. Carryover studies indicated a routine sample-to-sample interaction of less than 0.05% for both assays. 7. The digoxin assay routinely demonstrated intra- and inter-assay variations of less than 7.4% and less than 10.0%, respectively. The precision data for cortisol indicated intra- and inter-assay variations of less than 6.7% and less than 9.1%, respectively.