By spontaneous in vivo integration of a mat2:1 degrees plasmid, containing a Plus (P) cassette, into an h-L MT region of Schizosaccharomyces pombe an h+ strain was obtained which neither mutates to h- nor to h90. Southern blotting showed that it possesses the same mating-type (MT) configuration as h-S except that P information resides in both cassettes. Therefore the strain was called h+S. By crossing h+S with the h- strain LK42 of Engelke et al. (1987) it was possible to obtain h- recombinants with the MT configuration mat1:1(M)smt-o-L-mat2:3(P). Because of the totally defective smt signal (smt-o) in these recombinants no MT switching occurs, so that M information is conserved in mat1:1; furthermore the cassette mat2:3(P) is not expressed like in strains with a K region. This proves that the K region does not cause the silencing of mat2:3(P).