Yeast and horse alcohol dehydrogenases are commonly used as standards for radiation inactivation analysis of proteins, usually assuming that the minimal functional unit corresponds to the physical size in solution, a tetramer (Mr = 148,000) and a dimer (Mr = 80,000), respectively. Results described in this paper demonstrate that molecular weight overestimates may be obtained for the yeast protein as a result of its unusual sensitivity to secondary radiation products. Irradiation in the presence of sulfhydryl reagents results in a smaller functional size estimate (67,000 +/- 3000) than that obtained in their absence (128,000 +/- 5000), indicating that some sulfhydryl groups in the enzyme may be particularly susceptible to attack by radiolytic species. Analysis of the horse liver enzyme reveals that although it has structural and functional similarities to the yeast protein, it is not as prone to secondary radiation damage and gives a minimal functional size estimate (33,000 +/- 1000) that most closely corresponds to a monomer. Quantitation of disappearance of the protein from a sodium dodecyl sulfate gel as a function of radiation dose also gives a target size (48,000 +/- 3000) in reasonable agreement with the monomer molecular weight. These results indicate that the individual subunits of horse liver alcohol dehydrogenase have independent catalytic capacity and imply that the same may be true for the yeast enzyme.