Cholinesterase phenotyping is done at 25 degrees C with use of benzoylcholine as substrate and dibucaine, fluoride, chloride, and succinyldicholine as inhibitors. We wrote a diagnostic program in modified BASIC language for the processing and interpretation of these cholinesterase phenotypes. We used the HP 9830A calculator. The diagnostic aspect of the program uses 646 words, a further 86 words being used to run this program. The traditional and the programmed reporting procedures were duplicated for 296 consecutive patients, with no case of disagreement between the two reporting methods as to the appropriate phenotype. The programmed system requested nine repeat analyses more than did the traditional method. Seven of these were due to the tight limits set in the program for the fluoride inhibitor numbers. These tight limits were set to ensure that there was as limited an overlap as possible between the phenotypes E1uE1u and E1uE1f. The remaining two repeats were due to the restrictions placed on the succinyldicholine inhibitor numbers that would be acceptable before a patient was designated as having the relatively rare phenotype, E1aE1f.