The inhibiting effect of Ca on non-haem-Fe absorption is well established. Present studies showed that Ca inhibited haem-Fe absorption to the same extent when the same amount of Ca (165 mg Ca as CaCl2) was added to a meal. Attempts were made to examine the mechanism for this inhibition in the present studies. Meat is the only known dietary factor influencing haem-Fe absorption. The present studies were designed to examine whether Ca interfered with the enhancing effect of meat on haem-Fe absorption. We found that the inhibition was the same whether biosynthetically radio-Fe-labelled haemoglobin was given in meals with or without meat. The haem-Fe absorption ratio with:without added Ca was 0.59 (SE 0.07) when Ca was added to a hamburger meal, and 0.52 (SE 0.03) when added to a wheat roll. These values were not significantly different (t 0.95; P = 0.35). The inhibition of haem-Fe absorption by Ca is, thus, a direct effect on the absorption of haem-Fe and not an indirect counteracting effect of the well-known enhancing effect of meat on haem-Fe absorption. Control studies were conducted to ensure that haem-Fe had not been degraded to non-haem-Fe during preparation of the foods. Since Ca inhibits the absorption of haem- and non-haem-Fe to the same extent, the present results strongly suggest that Ca interferes with the transport of Fe through the mucosal cell, and at a late stage, is common for haem- and non-haem-Fe transport.(ABSTRACT TRUNCATED AT 250 WORDS)