Electrophoretic characterization of extracts from normal, leukemic and fetal mouse tissues indicated that infection with R L V results in the subsequent emergence of cells having profiles similar to fetal thymus and spleen tissues. Microdensitometric analyses showed diminution of prealbumin and haptoglobin esterase activity with concomitant predominance of postalbumin esterase activity in both fetal and leukemic thymus and spleen tissues. Both JLS V9 and J L S V6 infected cell cultures showed marked enhancement of postalbumin esterase activity. Leukemic and fetal thymus and spleen tissues also showed increased activity in LDH isoenzyme V, relative to the total L DH activity, compared to normal adult tissues. Increased relative activity in Band V LDH was also noted in infected V6 cells compared to uninfected cells. Increased amounts of proteins with electrophoretic mobilities similar to serum albumin and transferrin were evident in extracts of fetal and leukemic thymus and spleen tissues. Protein profiles of purified R L V showed a predominant band migrating similarly to serum albumin with minor bands migrating similarly to serum transferrin and haptoglobin.