We previously reported that two isoforms of intestinal alkaline phosphatase (IAP) are present in the serum, a high-molecular-weight isoform(HIAP) and a normal-molecular-weight isoform (NIAP), and that both are present at high levels in blood group B or O secretors. In the present paper, we investigated the relationship between effects of high-fat meal and blood groups on ALP activity. Subjects fasted for 14 hours after dinner the previous evening and ate a high-fat meal the following morning. Two types of meals were prepared; a low-calorie meal (470 kcal), and a high-calorie meal (950 kcal). Subjects ate the 2 types of meal on different days. Blood was collected 3 times; once preprandially, and at 3 and 6 h postprandially. Among B or O secretors (n = 24), the mean +/- SD for increase in ALP activity after the high-fat meal was 26.4 +/- 10.2 U/L and 23.3 +/- 9.0 U/L at 3 and 6 h postprandially, respectively, following the low-calorie meal, and 47.9 +/- 19.9 U/L and 55.1 +/- 21.9 U/L at 3 and 6 h postprandially, respectively, after the high-calorie meal. Thus, ALP activity increased 2-fold after the high-calorie meal. Similarly, among subjects with other blood groups (n = 28), the increase in ALP activity was 5.7 +/- 3.7 U/L and 4.2 +/- 3.1 U/L at 3 and 6 h postprandially, respectively, with the low-calorie meal and 8.5 +/- 5.2 U/L and 10.6 +/- 6.0 U/L at 3 and 6 h postprandially, respectively, with the high-calorie meal. Thus, significant differences were seen between the blood groups (p < 0.001). The increases in ALP activity after the high-fat meal were nearly identical to the increases in NIAP activity. These results suggest that a high-fat meal is more likely to affect ALP activity in blood group B or O secretors, and that this effect peaks between 3 and 6 h after the high-fat meal. Taken together, the present results indicate that, as a rule, blood samples for determining ALP activity should be collected in the early morning with the patient in a fasted state.