One hundred and twenty consecutive isolates of Neisseria gonorrhoeae from the County of Avon, UK, were examined by auxotyping and penicillin susceptibility testing. Requirements for proline (Pro), arginine (Arg), hypoxanthine (Hyp), uracil (Ura) and methionine (Met) were determined with a modified Heckels' medium. Prototrophic strains accounted for 29.9% of isolates and were a heterogeneous group as judged by penicillin susceptibility testing. The group most generally susceptible to penicillin required Arg-Hyp-Ura, and represented 27.5% of isolates. The Pro-Argo-Ura auxogroup (10% of isolates) had the narrowest range of penicillin susceptibility. Prototrophic strains had a reduced growth rate on arginine-free medium. Investigations of this phenomenon and suitable methods for auxotyping are reported.