  A model calculation for the reactions $pp\to p\Lambda K^+$ and $pp\to N\Sigma
K$ near their thresholds is presented. It is argued that the experimentally
observed strong suppression of $\Sigma^0$ production compared to $\Lambda$
production at the same excess energy could be due to a destructive interference
between the $\pi$ and $K$ exchange contributions in the reaction $pp\to
p\Sigma^0 K^+$. Predictions for $pp\to p\Sigma^+ K^0$ and $pp\to n\Sigma^+ K^+$
are given.
