Longitudinal non-load-carrying fillet welds in a structural steel were tested in axial tension. In constant-amplitude loading, the stress range approach was verified. In variable-amplitude block loading, stress interaction effects were observed. In general, interaction effects were beneficial or, in one case, only slightly detrimental to the fatigue life as compared to the Miner Palmgren summation index. For R = 0, interaction caused a significant delay in crack growth as compared to constant-amplitude loading, particularly with small cycles in the load spectrum. For R = −1, delay effects were small, and dependent on the sequence of peak/trough in each load cycle. Stress-relieved, as compared to as-welded specimens, exhibited a larger delay at R = 0, and a slight acceleration of crack growth at R = −1. Increasing the mean stress from 0 to 25 MPa gave only small changes, as did randomization of the block sequence.