In this report we further characterize the expression of the gene that encodes the 50-kDa spicule matrix protein (SM50) during development of the sea urchin Strongylocentrotus purpuratus. Quantitative measurements of SM50 mRNA levels using the single-stranded probe excess titration technique indicate that SM50 transcript levels attain a maximum level of 8000 to 10,000 transcripts per embryo by the gastrula stage, representing 120 to 200 SM50 mRNAs per primary mesenchyme cell. Experiments analyzing run-on transcription in nuclei isolated at different stages of development indicate that the sharp increase in SM50 mRNA levels occurring at the time of primary mesenchyme ingression is concomitant with an increase in transcription of the SM50 gene. We have also analyzed the RNA sequences present on polyribosomes at different stages of development. These studies indicate that SM50 mRNA is present on polyribosomes as soon as it begins to accumulate (which is well in advance of overt spicule formation) and SM50 mRNA remains on polyribosomes through subsequent development. From estimates of the rate of SM50 protein synthesis based on these data, we calculated that the maximum amount of SM50 accumulated during development through the 4-day pluteus stage is approximately 7.4 pg/embryo. This approximation is concordant with the amount of SM50 actually found in the sea urchin embryo.