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@Jfcarter,

Thanks for the answer, In the meantime i found that solution too, and works fine.

But it seems that this is not the re-captcha plugin what i meant. What i meant is the plugin from http://recaptcha.net/.

Now i have in register page wpmu-signup-captcha.php working, and in comments and posts wp-recaptcha…

Anyways if i find a better way i will post it here.