The slime mold Dictyostelium discoideum has two developmentally regulated forms of the enzyme glycogen phosphorylase. The inactive ’b’ form requires 5'AMP for activity and is present in early development, whereas the active ’a’ form is 5'AMP independent and is present in late development. Polyclonal antibodies raised to purified forms of the enzyme show low cross-reactivity. The anti-’a’ is specific for a 104 kd protein associated with phosphorylase ‘a’ activity; the anti-’b’ is specific for a 92 kd protein associated with the ’b’ activity. The two antibodies inhibit the activities of their corresponding antigens, furthermore, each antibody recognizes the proteolytic products of its corresponding antigen. In the presence of exogenously added Mn²⁺ and ATP, the ‘b’ form shows apparent conversion to a 5'AMP independent form as detected spectrophotometrically. This apparent conversion is accompanied by in vitro phosphorylation of the ’b’ enzyme by a Mn²⁺ dependent protein kinase. The ’b’ kinase also phosphorylates casein in the presence of Mg²⁺ or Mn²⁺. In vivo phosphorylation of the ’b’ form was observed in early development. Phosphorylation of the ’b’ form did not result in the appearance of the 104 kd protein. At this point, it is unclear whether Dictyostelium phosphorylase ’a’ represents a phosphorylated and activated form of the ’b’ form, or whether it represents a separate gene product. / Ph. D.