The hypobranchial gland of the muricid Plicopurpura pansa (Gould, 1853) is so active that the snails can be stimulated periodically without harming them to expulse the secretion. This property is a great advantage in the study about its biologic and chemical characteristics. No statistical difference could be determined between the incidence of expulsion and the sex of the animals. Also the test on whether the size of the animals had an influence on the frequency of expulsion showed no relation. The incidence of expulsions is the same between the different size classes. From between September 2003 and February 2005 collected snails (total number 3,577) 1,724 (48.2%) expulsed secretion. The proportion of snails that expulsed or not varied from month to month, however no clear seasonal trend could be observed. We determined in the laboratory the amount of the total organic compounds in the “milk” and found great variations (from 34.2 mg/100 animals to 337.8 mg/100 animals). We determined from 11 samples collected during different months an average of 148.9 mg organic compounds/100 animals. The “milk” expulsed from the hypobranchial gland of P. pansa contains 6.15% (±1.07 SD n = 3) total solids, 21.3 mg/ml (±17.8 SD n = 38) soluble proteins, and 6.01 mg/ml (±3.2 SD n = 38) carbohydrates. In organic extracts from the secretion of the hypobranchial gland we determined in a microwell assay a 50% lethal dose (LD50) of 81.72 μg/mL (SD 35.78 n = 5) against Artemia nauplii. In assays to determine possible antibacterial activities in organic extracts we found two inhibition zones against Staphylococcus aureus. To quantify the microbial activity we determined a lowest inhibitory concentration of 125 μg/disk. By thin layer- and column chromatography, as well as by IR spectroscopy, we could preliminary identify some of the organic compounds in the “milk” and in organic extracts. By comparing previously reported front reference values (Rf-values) we could identify the dye precursor tyrindolinone, tyriverdin and bromoisatin. By column chromatography the extract was separated with different mixtures of organic solvents. In a first step gradient fractionation we obtained three fractions, which were unstable in light and turned immediately purple, and one yellow, light stable fraction. Two light stable brownish-colored fractions turned purple after acid hydrolysis. They were united and subjected to further fractionation, where four fractions and the green insoluble tyriverdin were obtained. By IR spectrophotometry and comparison with reported spectra it could be shown that one fractionated compound was a salt of probably 6-bromo-2-methylsulfonylindoxylsulfate and the other 6-bromoisatin. In organic extracts of the secretion free radical scavenging activities were determined by the 2,2-diphenyl-1-picrylhydrazyl radical method (DPPH). We observed two yellow patches above a purple background. By IR spectroscopy of the organic extract used we could determine the chromogen IV, probably 6-bromo-2 methylsulfonylindoxylsulfate, as a substance responsible for the free radical scavenging activity.