The helicity, h -, of μ - in π-decay has been determined as positive (h -≥+0.90) from the average polarization, P av≡〈J B·s μ〉, of 12B produced in the μ -+ 12C→ν μ+ 12B reaction. We obtain also dynamical information on μ-capture: (i) the weak magnetism form factor, μ=4.5±1.1, and (ii) the sum of the induced pseudoscalar (g p) and the 2nd class induced tensor (g T) couplings versus g A, ( g P+g T) g A=7.1±2.7. The latter result, adopting the "canonical" value of g P g A, leads to g T g A=+1±2.7 which is compatible with zero and in strong contradiction with the value {reversed tilde equals}-6 recently advocated by Kubodera, Delorme and Rho. © 1977.