Expression of type IV collagen-degrading activity during early embryonal development in the sea urchin and the arresting effects of collagen synthesis inhibitors on embryogenesis. Type IV collagen-degrading activity was expressed in homogenates of Lytechinus pictus embryos during embryogenesis. Activity was concentrated 1,600-fold by ammonium sulfate fractionation, ion exchange, and gel chromatography and could not be activated further upon trypsin or organomercurial treatment. This enzyme activity could also degrade gelatin but had no affinity for type I, III, and V collagens. Activity was inhibited by addition of excess type IV collagen or gelatin, but was unaffected by addition of excess amounts of non-collagenous proteins of the extracellular matrix. Chelators such as 1,10-phenanthroline or Na2EDTA reduced activity to control levels. Inhibitors of plasmin and of serine and thiol proteases were without effect. Type IV collagen-degrading activity first became apparent at the stage of early mesenchyme blastula. It then increased by a small increment and remained stable up to the stage of late mesenchyme blastula, coinciding with first detection of collagen synthesis and the appearance of the archenteron. Thereafter, a sharp increase in activity was observed, concurrently with remodelling of the archenteron. Maximum activity was attained at prism stage and was retained throughout to pluteus-larva stage. The specific inhibitors of collagen biosynthesis 8,9-dihydroxy-7-methyl-benzo[b]quinolizinium bromide and tricyclodecane-9-yl xanthate arrested sea urchin embryo development at early blastula, prevented the invagination of the archenteron, and reverted the expression of type IV collagen-degrading activity to non-detectable levels. Removal of the inhibitors allowed embryos to gastrulate and express type IV collagen-degrading activity.