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Abstract. Somatomedin activity is often measured by 35SO4 uptake in pelvic chick embryo cartilage. This assay gives good results when the somatomedin activity is measured in serum, but often gives uninterpretable results for purified somatomedin. An ultrafiltrate (UF 1000) obtained from human normal serum, added to the incubation medium allows measurement of biological somatomedin activity even when somatomedin is highly purified. UF 1000 contains some compounds of molecular weight lower than 1000 daltons. UF 1000 acts either when used simultaneously with somatomedin or in pre-incubation with cartilage before testing biological activity. So, we chose to use it in pre-incubation (1 h) at the optimal concentration of 10% (v/v), the standard curves being realized with normal serum retentates. In these conditions, the proper action of UF 1000 does not interfere with calculation of somatomedin activity. This method has enabled us to show that a diminution of serum somatomedin activity can be linked either to an anomaly of UF 1000 or to a somatomedin deficiency.

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