Objective: The expression of contractile isoforms changes during various pathological conditions but little is known about the consequences of these changes for the mechanical properties in human ventricular muscle. We investigated the feasibility of simultaneous determination of protein composition and isometric force development in single cardiac myocytes from human ventricular muscle tissue obtained from small biopsies taken during open heart surgery. Methods: Small biopsies of about 3 mg wet weight were taken during open heart surgery from patients with aortic valve stenosis. These biopsies were divided in two parts. One part (~ 2 mg) was used for mechanical isolation of single myocytes and subsequent force measurement while the remaining part was used, in aliquots of I μg dry weight, for protein analysis by polyacrylamide gel electrophoresis. The myocytes were attached with silicon glue to a sensitive force transducer and a piezoelectric motor, mounted on an inverted microscope and permeabilized by means of Triton X-100. Force development was studied at various free calcium concentrations. Results: From all biopsies, myocytes could be obtained and the composition of contractile proteins could be determined. The average isometric force (±s.e.m.) at saturating calcium concentration obtained on 20 myocytes from 5 patients amounted to 51 ± 8 kN/m2. Force was half maximal at a calcium concentration of 2.47 ± 0.10 μM. Conclusion: These measurements indicate that it is possible to study the correlation between mechanical properties and protein composition in small biopsies from human ventricular muscle.