Proteins synthesized by Saccharomyces cerevisiae in presporulation and sporulation media were compared by using sporulating (a/alpha) and nonsporulating (a/a and alpha/alpha) yeast strains. Total cellular proteins were labeled with [35S]methionine and analyzed by two-dimensional polyacrylamide gel electrophoresis. Autoradiograms and/or fluorograms showed some 700 spots per gel. Nine proteins were synthesized by a/alpha cells which were specific to vegetative, log-phase conditions. During incubation in sporulation medium, sporulating (a/alpha) cells synthesized 11 proteins not present in vegetatively growing cell. These same 11 proteins, however, were synthesized by nonsporulating (a/a and alpha/alpha) cells on sporulation medium as well. Nonsporulating diploids (a/a and alpha/alpha) were also examined with the electron microscope at various times during their incubation in sporulation medium. Certain cellular responses found to be unique to meiotic yeast cells in previous studies were exhibited by the nonsporulating controls. The degree to which all cell types (a/alpha, a/a, and alpha/alpha) were committed to sporulation was also determined by shifting cells from sporulation medium to vegetative medium. Some commitment to the meiotic pathway was observed in both the a/alpha and the a/a, alpha/alpha cells.