Summary. Somatomedin-A (SM-A) was assayed by Hall's method using chick embryo pelvis. The relatively high inorganic S0 4 concentration of rabbit sera (1.5 mM) interfered with the estimation of the relative potency R of SM activity. Separation of the serum stimulation period from radioactive sulfate incorporation (post-incubation system) thus permitted this inconvenience to be eliminated. Using this technique we have estimated rabbit SM-A activity from birth to 180 days of age. The sera from 0 to 2-day old newborn rabbits inhibited 35 SO 4 uptake by chick pelvis cartilage. We observed a transition period between 3 and 5 days of age after which the sera obtained became stimulant. SM-A activity increased slowly from 0.5 to about 1 U/ml in the age period between 10 and 180 days, but there was considerable individual variation.Up to now, three kinds of chemically and biologically different somatomedins have been described (Uthne, 1973 ; VanWyk et al., 1973) : somatomedin-A (SM-A) which stimulates sulfate uptake by chick cartilage (Hall, 1970) ; somatomedin-B (SM-B) which stimulates DNA synthesis in human glia-like cells (Uthne, 1973), and somatomedin-C (SM-C) which stimulates sulfate uptake by rat cartilage (Van Wyck et al., 1974 Simultaneous incubation was carried out in a 20-hour water-bath, agitated for 30 s every 15 min. The post-incubation method demanded 20 hrs of incubation and 4 hrs of post-incubation. At the end of the incubation period, the leaflets were boiled, let stand overnight in saturated Na,SO,, rinsed for 3 hrs in running water, individually hydrolysed in 0.25 ml Soluene g (Packard) and measured for radioactivity in a classical scintillation medium PPO-POPOP-toluene.Statistical analysis. -Statistical analysis was done using covariance of the 4-point bioassay (Finney, 1964) ; for computation we used an HP 10 computer programmed for this operation.Results.Post-and simultaneous incubation. (Salmon, 1972 ;Garland et al., 1972). However, heat suppression of the inhibitory power of newborn rabbit serum is not a specific enough procedure to assign it to this category of proteic inhibitors.Since the heat treatment brings about a strong diminution of serum SM activity, a possible explanation could be that blood SM is protein-bound (Hintz, 1974a
