The cis and trans isomers of 3-MF were differentiated by their retention times and abundance ratios. Quantitation of powdered samples was conducted using GC/MS with an HP5 column and ketamine as an internal standard. Two street samples contained a higher percentage of the more potent cis isomer than the trans isomer. This drug is about 1000 times more potent than heroin; the difficulty of cutting the drug and inexperience with its strength has contributed to a rash of overdoses in at least one metropolitan area. Results highlight the need for thorough examination of residues in plastic bag corners and spoons which might alert the toxicologist to perform tests for this drug. 1 table, 3 figures, and 6 references (Author abstract modified)