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A single-stranded DNA probe complementary to chicken type II procollagen mRNA has been used to quantitate levels of that mRNA present in chicken limb mesenchyme during cartilage differentiation. Excess labeled probe prepared from a cDNA template cloned in M13mp9 was hybridized to completion to increasing amounts of total RNA and assayed by protection from S1 nuclease digestion. Estimates of the absolute levels of type II procollagen RNA were determined using the M13mp9 template containing the coding strand as a standard. RNA complementary to the probe increased from 20 copies per diploid genome in stage 24 limb to approximately 2000 copies per diploid genome in stage 24 limb mesenchyme which had differentiated to cartilage in culture. Similar levels were found in cartilage from stage 31 limb. Sternal cartilage from 17-day embryos contained approximately 10,000 copies per diploid genome suggesting that the level of expression of this gene is different in limb growth cartilage compared with sternal cartilage. Low but detectable levels of RNA complementary to the probe were observed in limb at stages 20-24. Since a large fraction of the type II procollagen RNA in these early limbs is associated with polysomes, the type II procollagen gene appears to be expressed at a low level prior to phenotypic differentiation and prior to the accumulation of immunologically detectable levels of type II collagen.