Recent studies have shown that the human endometrium is capable of synthesizing a 34 kd insulin-like growth factor-binding protein (IGF-bp). Serum levels of immunoreactive IGF-bp were measured in non-pregnant women in order to find out whether the uterus contributes to the circulating level of 34 kd IGF-bp and whether these values would reflect the endometrial function. Serum samples were collected from 21 women of presumably fertile age before and after hysterectomy, from eight apparently healthy women at various phases of the menstrual cycle, and from 23 post-menopausal women. The serum concentrations of IGF-bp were estimated by radioimmunoassay (RIA), which measured both the unsaturated and the IGF-bound 34 kd IGF-bp. After hysterectomy, a significant decrease in serum IGF-bp levels was observed in samples taken one week post-operatively suggesting a uterine contribution to the circulating IGF-bp concentration. Five weeks after hysterectomy the serum IGF-bp level had returned to the pre-operative level indicating that other tissues can compensate for uterine IGF-bp. In healthy women of fertile age the serum IGF-bp level (94 samples from eight subjects) was 39 +/- 1.8 ng/ml (mean +/- SE) and, in post-menopausal women (23 subjects), it was 36 +/- 6.0 ng/ml. During the menstrual cycle, the average day-to-day variation of serum IGF-bp concentrations was 31%. No correlation was found between the levels of serum IGF-bp and oestradiol, progesterone, LH or FSH. These results indicate that, while the uterus is probably one of the sources of circulating 34 kd IGF-bp, other tissues must also contribute.(ABSTRACT TRUNCATED AT 250 WORDS)