High resolution spectra of the B0.5 la supergiants-kappa Orionis, Sk-68-degrees-41 in the Large Magellanic Cloud (LMC) and Sk 159 in the Small Magellanic Cloud (SMC) are analysed using non-LTE model atmosphere and line formation calculations. The following stellar parameters are derived:[GRAPHICS]where y is the helium to hydrogen ratio by number. Abundances are also obtained for the elements C, N, 0, Mg and Si. The gross deficiencies in these elements found for Sk 159 and the more moderate deficiencies found for Sk-68-degrees-41 confirm the general findings of previous work for the SMC and LMC, respectively, although there are some differences in detail. The general helium enrichment of these stars is interpreted as evidence of enrichment of their atmospheres by CNO-processed material caused by mass loss and mixing processes during a previous red supergiant phase of evolution. Support for this hypothesis from the metal abundances is at present rather ambiguous due to a combination of observational and theoretical uncertainties. Additionally, some problems in the determination of the atmospheric parameters are discussed, in particular, those caused by the difficulties encountered in fitting the neutral helium lines. One consequence of these uncertainties is that we are unable to rule out the possibility of a normal helium abundance for kappa-Orionis.