Biochemical parameters of nutritional status were investigated in 300 women of Mexican descent during the first and second trimesters of pregnancy. Blood samples were obtained from the women during clinic visits, and measurements were made of serum iron, hemoglobin, hematocrit, serum protein, and transferrin saturation. Additionally, the nutritional status of seven vitamins was determined either by direct assay of the vitamin levels in blood or by measurement of erythrocyte enzyme stimulation. Thiamin and riboflavin were also determined in causal urine samples. Very few women, 8% or less, were classified as being low or deficient in hemoglobin, serum protein, iron, transferrin saturation, vitamin C, carotene, vitamin A, or vitamin B12. Thirty-one percent had low or deficient hematocrit values according to the guidelines used. Folic acid was the most prevalent vitamin deficiency, with 69% of the women having low or deficient serum levels. Based on the erythrocyte enzyme stimulation tests, 22% of the women were low or deficient in thiamin, 29% were low or deficient in riboflavin, and 9% were deficient in pyridoxine. None of the women had a low urinary excretion of thiamin, but 8% had excretion values of riboflavin below the acceptable level. Fewer deficiencies of thiamin and serum folic acid were observed in women taking vitamin and mineral supplements than in those who were not.