The radiation inactivation method was used to determine the molecular size of the two enzymes that participate in the synthesis of the phosphomannosyl recognition marker of lysosomal proteins. The determinations were carried out in situ, in Golgi membranes isolated from normal human placenta and cultured skin fibroblasts. A molecular size of 228 +/- 29 kDa was found for placental N-acetylglucosaminyl-phosphotransferase, and 129 +/- 11 kDa for placental alpha-N-acetylglucosaminyl phosphodiesterase. The values for the fibroblast enzymes were about 20% higher, 283 +/- 27 kDa and 156 +/- 14 kDa for the transferase and phosphodiesterase respectively. Triton X-100 had no effect on the molecular size of these enzymes.