Mutant Ts/+ fetuses are developmentally retarded as compared to normal +/+ littermates. Mutant fetuses have less total hemoglobin than do normal fetuses of the same gestational age. However, when compared to +/+ fetuses of similar body weight, Ts/+ fetuses have the appropriate amount of total hemoglobin, suggesting that the apparent anemia observed in mutant fetuses is most likely the result of delay in growth and development. Changes in proportions of embryonic hemoglobins during fetal development are similar in Ts/+ and +/+ fetuses at day 12 and later of gestation. Moreover, adult hemoglobin is detected in circulating primitive nucleated erythrocytes in the developmentally retarded Ts/+ mutant fetuses at about the same chronologic age as their +/+ normal littermates.