The effects of HgCl(2), CuCl(2),CdCl(2) and ZnCl(2) on the fertilization, cell division and pluteus formation of eggs from the sea urchins, Hemicentrotus purcherrimus and Temnopleurus toreumatics, were studied. Inhibition of the development of sea urchin eggs by these heavy metals was ameliorized by slag from both paper manufacturing effluent and the ended at the river mouth. The development of sea urchin eggs was completely inhibited by HgCl(2) at 10(-6)M. A concentration of 10(-4)M was needed for this inhibitory effect after HgCl(2) had been mixed with slag. Abnormalities of larvae were not observed with a concentration of 10(-4)M. The critical concentration of CuCl(2) was 10(-5)M. This dose to 10(-4)M after treatment with slag. CdCl(2) caused complete inhibition of spicule formation at 10(-5)M, resulting in the formation of permanent blastulae. A concentration of 10(-4)M was needed to obtain the same effects after slag treatment. The critical concentration of ZnCl(2) was 10(-5)M. The same effect was obtained with this concentration even after the addition of slag from river mouth slag, whereas after the addition of paper mill effluent, a concentration of 10(-4)M was needed.