The low mol wt heat shock protein (hsp27) exists as three forms. hsp27a is the most basic and is not phosphorylated. The other two forms are phosphorylated, with hsp27b of intermediate isoelectric point and hsp27c as the most acidic form. We hypothesized that a Sertoli cell protein, previously discovered as being responsive to germ cells and referred to as GC1, is hsp27c. To investigate this hypothesis, cultured Sertoli cells were treated with CdCl2 or were heat shocked and then labeled with H3(32)PO4, [35S]methionine, or 3H-labeled amino acids. Sertoli cell proteins were then prepared for two-dimensional polyacrylamide gel electrophoresis and autoradiography or fluorography. The 27K protein did not incorporate [35S] methionine. After CdCl2 or heat shock treatments, hsp27b and hsp27c (GC1) showed increasing 32P incorporation with time. To test if this result was due to increased synthesis of hsp27 Sertoli cell proteins were labeled with 3H-labeled amino acids. Heat shock and CdCl2 resulted in time-dependent increases in 3H label incorporation into all three forms of hsp27. Shortly after heat shock, a greater proportion of hsp27b and hsp27c was observed in the pellet fraction of Sertoli cell homogenates, with more of these hsp27 forms being found in the supernatant fraction after 4 h recovery from heat shock. Treatment of CdCl2-exposed Sertoli cells with germ cells increased 32P labeling of hsp27c (GC1). The location on two-dimensional gels of the most acidic form of hsp27, hsp27c, was exactly coincident with the location of GC1. Taken together, the results support the conclusion that GC1 in Sertoli cells is hsp27c.