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To investigate the potential of chemotherapeutants to control the oyster pathogen Perkinsus marinus, anticoccidial and antifungal compounds were tested in vitro on infected hemolymph and cultured P. marinus cells and in vivo on infected oysters. In addition, acute toxicity to oysters was determined for six anticoccidials. In vitro experiments with infected hemolymph consisted of 24 h exposure of 0.2 mL hemolymph aliquots to concentrations ranging from 100 mg/L to 0.01 mg/L of amphotericin-B, amprolium, arprinocid, cycloheximide, lasalocid, malachite green, monensin, sulfadimethoxine, and a potentiated sulfadimethoxine, followed by incubation in fluid thioglycollate medium (FTM) to determine prezoosporangia abundance. Lasalocid, malachite green, and amphotericin-B were the most effective compounds reducing prezoosporangia abundance, relative to the untreated control group, at concentrations as low as 10 mg/L. Cycloheximide, monensin, and to a lesser extent sulfadimethoxine, were also effective but only at the highest concentration tested (100 mg/L). In vitro experiments with cultured P. marinus consisted of 24 h exposure of 10&\sp5& cells to 100 mg/L, 10 mg/L, and 1 mg/L of amphotericin-B, and 100 mg/L of cimetidine, cycloheximide, fumagillin, 5-fluorocytosine, ketoconazole, lasalocid, and monensin, followed either by incubation in FTM to determine abundance and size of prezoosporangia, or by addition of Neutral Red to determine cell viability. Amphotericin-B, lasalocid, and monensin were effective in reducing prezoosporangia abundance, size, and/or cell viability. No effects of cycloheximide on cultured cells were apparent. Lasalocid, monensin, and malachite green, were toxic to oysters at concentrations below 10 mg/L. The 96-hr. LC50 for lasalocid was 0.59 mg/L. No median lethal dose was determined for monensin or malachite green, but oyster mortality resulted from exposures ranging from 1 mg/L to 10 mg/L of either compound. In three in vivo experiments, infected oysters were exposed to amprolium, arprinocid, cycloheximide, lasalocid, monensin, malachite green, potentiated sulfadimethoxine, and sulfadimethoxine at various concentrations. Only cycloheximide was effective in reducing P. marinus infections. After 15 days of exposure to 10 mg/L of cycloheximide, weighted prevalence significantly declined from 3.78 in untreated controls to 2.10 in treated oysters. Infections progressed after treatment was discontinued as indicated by an increase in weighted prevalence from 0.71 at the end of treatment to 1.31 one month later. (Abstract shortened by UMI.).

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