We evaluated two different assays for the determination of bone-specific alkaline phosphatase (B-AP). One assay was a direct method using a two-site IRMA (AP(Hyb)). The other assay determined B-AP indirectly after precipitation with lectin (AP(BM)). The assays were compared to serum osteocalcin (levels) and total AP in 20 premenopausal women and in 40 early postmenopausal women before and after 9 months of treatment with hormone replacement therapy (HRT) (n = 20) or placebo (n = 20). Serum osteocalcin correlated with serum AP(Hyb) (r = 0.45), and with serum AP(BM) (r = 0.33) (both p < 0.001). The correlation between AP(Hyb) and AP(BM) was moderate, r = 0.71 (p < 0.001). When comparing postmenopausal to premenopausal women, serum osteocalcin had a t score (mean +/- SEM) of 2.30 +/- 0.29 followed by total AP(T-AP) (1.20 +/- 0.28), AP(Hyb) (1.05 +/- 0.33) and AP(BM) (0.64 +/- 0.21) (paired t-test: p < 0.01 for osteocalcin vs. other markers). After 9 months of HRT all markers had declined significantly to premenopausal levels (mean +/- SEM): osteocalcin with 44.5% +/- 6.1%; AP(BM) with 32.1% +/- 5.6%; AP(Hyb) with 36.4% +/- 4.4% and T-AP with 24.4% +/- 2.9%; p < 0.001. Of the markers, only serum osteocalcin correlated significantly with the rate of bone loss in the placebo group (r = 0.52, p < 0.02). We conclude that both assays for B-AP reflect bone turnover in postmenopausal women with and without HRT. Under controlled conditions they did not show any advantage of osteocalcin in diagnosing increased bone turnover or monitoring the effect of an anti-resorptive therapy in postmenopausal women.