Lettuce, in its cultivated form (Lactuca sativa L.), is a crop of economic importance with several characteristics which make it well suited for biochemical and genetic studies. Biochemical traits such as proteins and enzymes have been studied extensively in many plant species and constitute an important experimental approach to physiological, evolutionary, taxonomic and breeding studies. Seed proteins were extracted from an array of lettuce cultivars. Polyacrylamide gel electrophoresis was employed in a comparative analysis of the soluble protein and isozyme characteristics of seed from each cultivar. Some fall-desert, winter-desert, and coastal cultivars were distinguishable based upon variation in soluble proteins or esterase isozymes. Multiple forms of the carboxylic ester hydrolases or esterases have been shown to occur in a wide variety of plants; their role in the plant cell is still however poorly understood. The inheritance of the esterase isozymes was analyzed by a microelectrophoresis technique which enabled the analysis of individual seeds in the progeny from a cross involving two winter-desert cultivars of contrasting banding phenotypes. Two distinct banding patterns were observed in single seeds of these cultivars; their F₁ hybrid showed a summation of parental patterns, the F₂ segregated in a 1:2:1 phenotypic ratio for these esterase patterns, and backcross segregation ratios were 1:1. Banding pattern differences could be accounted for by the segregation of a single gene with codominant action. Quantitative as well as qualitative differences in esterase activity were observed between the parental lines from two different years of production. Some esterase isozymes were developmentally regulated. Molecular weight determination experiments verified the presence of two gene products of 56 and 62.5 Kd.