We describe the usefulness of a fast (48-h) limiting dilution assay (LDA) for the enumeration of human alloreactive helper T lymphocytes (HTL) in the peripheral blood, in relation to histologically defined rejection grades after heart transplantation. HTL frequencies (HTLf) in pretransplant samples varied from patient to patient, ranging from 106 to 625 HTL/106 peripheral blood mononuclear cells (PBMC). In the first week after heart transplantation (HTx), when immunosuppression was instituted, HTLf were significant lower (range 30-190 HTL/106). The level of HTL in the first week after HTx when rejection grade was 0 or 1A (ISHLT score) was considered to be the baseline frequency. This frequency did not correlate with the number of subsequent rejection episodes. During rejection (grade 3), donor-specific HTLf were increased above their baseline frequencies (P = 0.01). Expressed as percentage of baseline frequencies, HTLf increased significantly during acute rejection (AR) compared with 1-2 weeks before rejection (P = 0.003). The increase was specific, since viral infections did not result in a rise of donor-specific HTL, while also HTLf specific for third party HLA antigens were not elevated during rejection. Monitoring HTLf in peripheral blood with a shortened (48-h) assay may serve as a non-invasive method for detecting intragraft immunological reactivity. Demonstrating absence of donor-specific reactivity may limit the number of invasive endomyocardial biopsy (EMB) procedures and allow tapering of immunosuppressive treatment.