Pseudopterosin A is a diterpene riboside isolated from Pseudopterogorgia bipinata, a soft coral of the order Gorgonacea. Pseudopterosin A inhibits the first cleavage in fertilized sea urchin eggs (Strongylocentrotus purpuratus, Strongylocentrotus franciscanus, and Lytechinus pictus) with an IC50 of 25 microM. In time of addition studies, Pseudopterosin A (4 X 10(-5) M) progressively blocked the first cleavage when added within the first 50 min post fertilization; when added after 50 min the first division occurred normally. Cell cycle studies show that Pseudopterosin A had to be in continuous contact with the sea urchin eggs during the initial 45 min post fertilization to significantly inhibit the first cleavage. Shorter drug contact time progressively reduced the degree of inhibition, suggesting that inhibition of cytokinesis by Pseudopterosin A was correlated with mitosis. Pseudopterosin A (4 X 10(-5) M) inhibited the uptake and incorporation of [3H]thymidine during the S2 phase. This concentration of Pseudopterosin A also inhibits protein synthesis as measured by the uptake and incorporation of [3H]phenylalanine. In this case the inhibition started before the S1 phase. Cytological examination revealed that sea urchin embryos did not progress beyond early prophase. Notably, the nuclear envelope remained intact and chromatin was condensed into chromosomes in the arrested embryos. These synchronously dividing embryos did not show any abnormalities such as lysis, swelling, or morphological changes different from control embryos.