An high-performance liquid chromatography (HPLC) procedure was developed for 1-aminocyclopropane-1-carboxylic acid (ACC) quantitation in banana fruit. This alternative method overcame difficulties encountered in previously published, nonHPLC procedures, and it is comparable to another HPLC procedure published recently. HPLC in a linear gradient mode yielded baseline separation of ACC from other amino acids. It was found that ACC content decreased concomitantly with an increase in ethylene (C[subscript]2H[subscript]4) production during the early stages of banana ripening. Conversely, a subsequent decrease in C[subscript]2H[subscript]4 production may have been caused by decreased ACC content or some unknown mechanism;Formation of 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC) does not seem to contribute to decreased ACC content, because MACC content, relative to ACC content, was small. MACC content was greatest when C[subscript]2H[subscript]4 production was greatest, and therefore, C[subscript]2H[subscript]4 seems to induce MACC formation. Malonic acid was the only conjugate of ACC that we found in banana fruit, and conjugation with malonic acid occurred in fruit of all ages. MACC content decreased as fruit aged, and this suggested that hydrolysis of MACC to ACC and malonic acid had occurred. However, the small amount that was hydrolyzed to ACC did not seem to contribute to an increase in ACC content;Respiration increased after the first peak of C[subscript]2H[subscript]4 production, and it increased concomitantly with the second peak of C[subscript]2H[subscript]4 production. This suggested that as long as respiration was not impaired, biological activities persisted and impairment of ethylene-forming-enzyme was retarded. Therefore, C[subscript]2H[subscript]4 production could increase during the later stages of ripening.