Second-class currents

Recent nuclear $\beta$-decay experiments have hinted at the existence of weak second-class currents. We review the evidence for second-class effects in other branches of weak-interaction physics and find that the presently available data are inconclusive. We then propose additional tests, involving (3,3)-resonance production by neutrinos and antineutrinos. The effects here could be quite substantial if the second-class interactions are as big as suggested by the nuclear $\beta$-decay experiments.