The relative rate of de novo synthesis of two membrane-associated carbohydrate-binding proteins (CBP) has been examined during Dictyostelium development. The results show that the relative rate of CBP synthesis is minimal during the vegetative stage and increases to represent approximately 3.5 to 5% of newly synthesized protein during the aggregation stage after which the relative rate decreases. Analysis of the relative rates of synthesis of CBP-26 and CBP-24 indicate that at the peak period of synthesis (approximately 5 to 9 h of development) CBP-26 is synthesized at a rate which is approximately eight times greater than CBP-24. In addition, we have examined the relative amount of CBP-26 and CBP-24 mRNA during development as assayed by its ability to direct CBP synthesis in in vitro protein-synthesizing systems. We show that there is no detectable CBP mRNA in vegetative cells and that during the pre-aggregating stages, assayable CBP mRNA appears and accumulates with a maximal level at the period of peak in vivo CBP synthesis. These results suggest that the rate at CBP synthesis in vivo is controlled by the relative amount of functional mRNA.