Efficient extraction and accurate analysis of lipophilic brevetoxins (PbTxs), produced by the harmful algal bloom (HAB) species Karenia brevis, are essential when assessing the toxicological potential of this dinoflagellate. One of the most commonly used brevetoxin extraction methodologies employs C18 solid-phase extraction (SPE). In this study, C18 SPEC discs were tested for extraction of spiked PbTx-3 in seawater and naturally produced brevetoxins from K. brevis. Quantification of brevetoxin in the extracts was determined using four independent methods: receptor binding assay (RBA), radioimmunoassay (RIA), neuroblastoma (N2A) cytotoxicity assay, and liquid chromatography/mass spectrometry (LC/MS). In addition to quantification of the brevetoxin concentration, LC/MS analysis provided identification of individual congeners and each of their hydrolyzed products. SPEC disc extractions prepared from sonicated cultures of non-brevetoxin-producing Karenia mikimotoi cultures spiked with PbTx-3 yielded extraction efficiencies of 108, 99, and 125% as determined by the RBA, RIA, and N2A cytotoxicity assay, respectively. In SPEC disc extracts of brevetoxin-producing K. brevis (isolate SP3) cultures, LC/MS analysis yielded the highest total concentrations, possibly due to the concurrent detection of hydrolytic brevetoxin congeners that accounted for up to 20% of the congener profile. Relative to the brevetoxin concentration as determined by LC/MS, the RBA, RIA, and N2A cytotoxicity assay detected 73, 83, and 51% of the total brevetoxin concentration. Stability experiments demonstrated that brevetoxins remain stable on the SPEC discs for at least 30 days, making this extraction method suitable for shipboard collections.