The plasma concentrations of protein-bound iodine (PBI), cholesterol and phospholipid were studied in British (Shorthorn × Hereford) and zebu cross (Brahman × British and Africander × British) cattle grazing in a tropical environment. In 20-month-old cattle, plasma PBI, cholesterol and phospholipid levels were higher ( P < 0.01) in zebu cross than in British animals and higher ( P < 0.01) in females than in males. Plasma PBI levels were positively correlated ( P < 0.001) with both cholesterol and phospholipid levels. Within sexes, the lipid level at a given PBI level was higher in zebu cross than in British animals. In pregnant cows the correlations between PBI and cholesterol levels were significant ( P < 0.001) but the relationship was negative in British animals. There were significant correlations between blood parameters of dams and birth-weight of calf in the British and Brahman-cross breeds. The results emphasize the importance of taking account of breed and sex differences when studying the blood chemistry of cattle in tropical areas.