The cellular slime mold, Dictyortelium dixcoideum, provides an ideal model system to study eukaryotic cell differentiation. In D. discoideum glycogen degradation provides precursors for the synthesis of developmentally regulated structural end products. The enzyme responsible for glycogen degradation, glycogen phosphorylase, exists in active and inactive forms. The activity of the ’a' form (the active form) is independent of 5′adenosine monophosphate (5′AMP) while the activity of the ’b' form (the inactive form) is 5′AMP dependent, The two forms are developmentally regulated. Polyclortal antibodies raised to the purified forms of this enzyme show low cross reactivity. The anti-’a' antiserum reacts with a 104 kd protein that is associated with phosphorylase ’a' activity; the anti-’b' antiserum reacts with a 92 kd protein that is associated with phosphorylase ’b' activity and cross reacts weakly with the 104 kd protein. Cyclic AMP perturbation of intact cells caused induction of both phosphorylase ’a' activity and the appearance of the 104 kd protein. lmmunotitration data suggest that the ’a’ form accumulates due to de novo protein synthesis, although this result must be interpreted with caution. In vitro translation experiments indicate that separate mRNA species exist for the two forms of phosphorylase. The mRNA for the 'b’ form is present throughout development while that of the ’a' form appears late in development.